Could you elaborate on what you mean by forgery? I mean, obviously historians don't generally believe it was literally written by the ibis-headed Egyptian god of writing and magic, Thoth.

Nor is the late Medieval/Renaissance figure Hermes Trismegistus who Thoth morphed into (via being equated with Hermes by the Greeks) considered to have been an actual historic figure. But a misattribution isn't really forgery.

Or do you mean the dating and origin? It does not come from Ancient Egypt. It (or rather parts of it; depending on the manuscript) did however originate in the Greek culture of Roman Egypt.