Dear instance admins,
I do not recommend blocking Gab. Block individual users if they are a problem, even Gab has some cool people on it.
Even if you must block Gab, I definitely do not recommend blocking instances for the sole reason that they do block Gab as well.
I hope you are realizing you are creating a massive split in the fediverse by doing so, especially to the apolitical instances like the one I'm posting this from, we do not block Gab or any other instance.