The GL Post Date is a required field for any gift but is also extremely important if your database is integrated with a Financial Product. The GL Post Date will be what appears with the gift in the Journal Entry in the Financial Edge regardless of the day the gift is posted from the Raiser's Edge to the Financial Edge. 

For more information on posting to the General Ledger, please reference How to post from the Raiser's Edge to Financial Edge (General Ledger)