Well, any GIA report is given a unique ID number. So if the report is fake, you can always go to GIA and ask for a cross reference of your report number. Also, GIA can laser inscribe your diamond so that it’s always marked with its unique ID number – no one can ever fake that diamond again.
GIA is pretty much the gold standard when it comes to diamonds (I’ve been shopping lately so I would know haha) and I would say good luck to anyone trying to counterfeit because everything can always be verified by GIA!