Maybe Trump needs some other way to communicate with the people that doesn't enrich a bunch of fucking evil assholes.
That is not Trump posting there, it is an account someone made and holds in reserve for him, should he decide to move to GAB. The person handling that account reposts everything from Trump's account and of course gab would not remove his video.
It is his, but he hasn't taken control of it and as it only retweets twitter shit, it wont be doing anything.
Let all the walnut sauce surround me dear baal