The main reason they banned "adult content" was simply that porn posters will fill any available online free forum. Reasonably enough, as a forum aimed at free speech, Gab wasn't eager to pay the hideous storage and networking costs of becoming another site like Tumblr, where that content had simply taken over...
Not if handled in the current social media "standard" way: Twitter, Reddit (and ex-Tumblr) keep nsfw content from sight of non-nsfw interested users by special handling - semi-hiding the category, won't show up on searches etc but can be linked to if you have nfsw opt-in.
They're not a free speech network, they're just a network where racism is permitted speech and anything they don't like is banned.