The Gift Date and GL Post Date options are used to determine which Gift record date field should be used to match the date range criteria of the report. 

Gift Date:  This option is used to report on all gifts that were made during the specific date range. 

GL Post Date:  This option is used to report on all gifts that were posted from Raiser's Edge to the General Ledger during the specific date range.  

For more information, refer to the Reports Guide (PDF) and the Sample Reports Guide (PDF).