is there a clear difference and if so what?
My Brill dictionary says that the perfect active is usually used in the sense “to be born” (to have been born) and cites passages from Homer to Aristophanes. It then gives the sense “to be created” (Clem.) which uses the perfect middle participle.
Does your question have a context?
It’s tricky. While they both function as the perfect of γίγνομαι, there are a number of variables controlling usage: chronological, dialectal, generic, stylistic, semantic.
γεγένημαι tends to be used absolutely, in the sense either “has come into being” or else “happened in the past and is now over” (e.g. “the past” is τα γεγενημένα).
γέγονα more likely with predicate, “I have become” (a god, depressed, whatever), or absolutely, “(something) has happened.”
Or that’s my off-the-cuff rough sense of it.