Afaik, Gab doesn't use Mastodon anymore. They did start as a Mastodon instance, but after being blocked from most instances and even at the software level by most Mastodon apps, they stopped using Mastodon and I believe wrote their own new backend that does not federate.
Here's the commit that removes the Public/All timeline (the above image mentions that removal): https://code.gab.com/gab/social/gab-social/-/commit/7ad7fe06...
Couldn't find when exactly Federation capability was removed, since that's less obvious from the commit messages (as someone who doesn't know Mastodon internals).
It's still based on the Mastodon fork, but they seem to have been changing the code constantly even before this (based on information under #gab on Mastodon), so it's not clear how much it resembles the Mastodon source by now.
PS: Somehow looking at the source repo and commit messages makes this more "real" for me, more than any screenshots or the user-facing site itself. It's a weird feeling.
In a way that already happened. The biggest Mastodon instance is now the infamous 'alt-right' 'chan'-like community Gab.
That you haven't even noticed it shows that Mastodon might be immune to "Eternal September" issues.