It absolutely, 100% is Gab's responsibility for not policing their platform. In fact, they invite this kind of behaviour. This is exactly what they want on their site.
Same way Google isn't held liable for hosting illegal pirated content as long as they take it down. But you can't go sue Google because they show you news about something which you find offensive.
They invite the worst kind of hatemongers to their site. That makes them as bad in my books. If all you care about is whether they are following the letter of the law or not, that is a very naïve and childish view.
What about all the hate towards Trump when he says good morning, have a nice day? Nothing happens.
Of course, we know why. It's because the people who run them are fine with the right kind of hate, if they believe it to be justified by history.
Yes, the law codifies a unified set of rules and definitions. Morals and ethics do not have that quality.