Because the criticism by mainstream media obviously isn't that they're cooperating with law-enforcement (if they are), but that they're very willing to host a lot of the awful stuff.
Details matter. They alerted law enforcement to Robert Bowers' profile before the police or the public knew about it, and certainly before criticism from the mainstream media. They could quite plausibly covered this up seeing Gab.com does not have a wide user base.
The profile had been discovered before they removed the contents. A few people / journos downloaded the contents. So no, they couldn't have covered this up.
The full profile had been given to law enforcement before its removal from public view. It was removed, as is routine for such profiles, in order to prevent copycat murders.