Whoever it is, the tweets do not excuse the nature of the nastiness. I thought it was funny at first but it's escalated out of hand now.
This can be approached in two different ways (both similar):
Is it definitely Gabby? - If so, then he needs to stop what he's doing immediately and delete his Twitter before either the club, the media or the police pick up on it. I'm surprised no one has grassed him up yet, especially the guy who is pissing him off.
Is it an imposter? - If it is an imposter using videos from Gabby then they need to be reported to the police straight away. Gabby needs to be told about it (maybe via the club) and his "mate" either needs to stop **** around and get a bollocking and of course, delete the account. The club probably won't put out a statement saying it's a fake account because it's private and doesn't have Gabby's name on it, so that's his get out clause, I guess. Either way, something needs to be done.
Personally, I think it is Gabby. It's irrelevant whether or not any players have interacted with Gabby on Twitter, it's up to whoever is controlling the account to give access to individuals, and maybe Gabby has opened the account not to have that interaction with them?