Gab actually uses a custom user agent to grab stuff.

Mastodon does the same, but this makes it easy to make sure gab instances can't pull from you!

add
if ($http_user_agent ~* (gabsocial)) {
return 403;
}

To your nginx configuration. That blocks all requests with a user agent that includes "gabsocial" (case insensitive)

You can also add other user agents, at your leisure

It just returns a 403 if they try to pull