Both Gabriel is superior. In show and character.
Of the two I'm going to say Umaru just because Gabu has only had one episode. But neither is best girl.
Umaru because that looks like a stupid slut.
Gab is boring and overshadowed by other characters in her show. Umaru easily carries her anime by herself.
Gabriel is okay, but Umaru feels like she was literally designed for using to create forced memes and shitposting.
Gabriel is superior.