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 and the Gift Date will default to the day the record was created. That date value is pulled from the regional settings on the workstation that houses the database files. 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.