Yeah, the sign-up process can be a hassle with gitter (e.g. you must have a github account). However the real advantage I see is that the gitter conversation is public, so you don't really need to go through the sign-up process unless you want to contribute to the discussion. And even if you wish to contribute, you just have to be on github (no need to ask for invites).
I, for one, use both slack and gitter everyday (mostly slack). However, I feel that slack is better suited for private uses (like a company) while gitter is more community oriented.