The shooter's content was scrubbed and gab notified the FBI once they were made aware of it. What else were they supposed to do?
Is there any chance they don't yet have the tools to police it? It's taken a fair amount of time and resources for other platforms to do it, to the point that YouTube/Twitter rely on users to do so.
Again, seems to me we're holding to different standards.
If you read interviews with Gab’s owner you will see there is no intention to mitigate the site’s extremism or radicalization of its members toward violence.