Can a GC hold retention from one job location for another location of work performed in Georgia?

Unless they have a contract provision allowing them to hold money on one job for an unrelated project, I would argue they cannot do that. The law does provide for a person to defend a lawsuit by arguing they are "setting off" a debt against another amount due, but Georgia has prompt payment requirements and the contractor will be playing with fire.

If I understand your question, the GC is withholding retainage due to you from another job that has been completed, correct? It sounds like the GC is exercising a setoff. Absent an agreement to the contrary between you and the GC, the GC cannot withhold payment you have earned from another job. If you're still within your 90-day lien period, you can file a lien against the other project for which you've completed your work.