I don't disagree about Gab being far-right, but antisemitism is an equal-opportunity form of xenophobia. There's both left-wing and right-wing antisemitism.

In the US, the right wing is often bound up with co-opting Jews via the term "Judeo-Christian" and supporting Israel, even it its worst abuses of the Palestinians. They often call-out left-wing antisemitism, which often tries to take the form of "anti-Zionism" (but whose information is often traced to antisemitic channels).

So antisemitism doesn't by itself demonstrate far-right leanings -- even though the far right is also full of literal Nazis and those who call them "very fine people". But Gab is certainly happy to play host to those, too.