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Subsequent Events - Monthly Distributions Declared by Company's Board of Directors (Detail) (USD $)
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0 Months Ended |
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Jan. 13, 2015
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| Subsequent Event A [Member] |
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| Dividends Payable [Line Items] |
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| Record Date |
Jan. 23, 2015
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| Payment Date |
Feb. 03, 2015
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| Payable to the Holders of Records During the Month |
January
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| Subsequent Event A [Member] | Common Stock [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1250000us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventOneMember
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| Subsequent Event B [Member] |
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| Dividends Payable [Line Items] |
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| Record Date |
Feb. 18, 2015
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| Payment Date |
Feb. 27, 2015
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| Payable to the Holders of Records During the Month |
February
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| Subsequent Event B [Member] | Common Stock [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1250000us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventTwoMember
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| Subsequent Event C [Member] |
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| Dividends Payable [Line Items] |
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| Record Date |
Mar. 20, 2015
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| Payment Date |
Mar. 31, 2015
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| Payable to the Holders of Records During the Month |
March
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| Subsequent Event C [Member] | Common Stock [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1250000us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventThreeMember
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| Subsequent Event [Member] | Common Stock [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.3750000us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
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| Senior Common Stock [Member] | Subsequent Event A [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.0875us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventOneMember
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| Payment Date |
Feb. 06, 2015
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| Senior Common Stock [Member] | Subsequent Event B [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.0875us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventTwoMember
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| Payment Date |
Mar. 06, 2015
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| Senior Common Stock [Member] | Subsequent Event C [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.0875us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventThreeMember
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| Payment Date |
Apr. 07, 2015
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| Senior Common Stock [Member] | Subsequent Event [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.2625us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= good_SeniorCommonStockMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
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| Series A Preferred Stock [Member] | Subsequent Event A [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1614583us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventOneMember
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| Series A Preferred Stock [Member] | Subsequent Event B [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1614583us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventTwoMember
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| Series A Preferred Stock [Member] | Subsequent Event C [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1614583us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventThreeMember
|
| Series A Preferred Stock [Member] | Subsequent Event [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.4843749us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
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| Series B Preferred Stock [Member] | Subsequent Event A [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1562500us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventOneMember
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| Series B Preferred Stock [Member] | Subsequent Event B [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1562500us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventTwoMember
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| Series B Preferred Stock [Member] | Subsequent Event C [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1562500us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventThreeMember
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| Series B Preferred Stock [Member] | Subsequent Event [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.4687500us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
|
| Series C Preferred Stock [Member] | Subsequent Event A [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.1484375us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesCPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventOneMember
|
| Series C Preferred Stock [Member] | Subsequent Event B [Member] |
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| Dividends Payable [Line Items] |
|
| Distribution per Share |
0.1484375us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesCPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventTwoMember
|
| Series C Preferred Stock [Member] | Subsequent Event C [Member] |
|
| Dividends Payable [Line Items] |
|
| Distribution per Share |
0.1484375us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesCPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= good_SubsequentEventThreeMember
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| Series C Preferred Stock [Member] | Subsequent Event [Member] |
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| Dividends Payable [Line Items] |
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| Distribution per Share |
0.4453125us-gaap_DividendsPayableAmountPerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesCPreferredStockMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
|