GL post date is located on the Miscellaneous tab of a gift record. This date is what will appear in the General Ledger posting as the transaction date for the gift. For additional information on this field, please refer to the following: What is the GL Post Date?
When entering a new gift, the GL Post Date will default from the value entered for the Gift Date. If the dates need to be different, manually edit the fields.
Regarding "which date" to enter into these fields, please consult with your accounting, bookkeeping, auditors, and/or other financial advisers.