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.
First, I 100% agree with you that what the mainstream right categorizes as the "far left" is really anyone "center left" that they disagree with (or even slightly right, as many mainstream neoliberal Democrats fall on the "political compass" (though take that with a grain of salt)).
However, there are absolutely folks on the far left advocating for violence as a means to their goals. I see more of them online, obviously, than in real life (though I do know folks who do black bloc, destroy property, etc.), but it's really hard to know to what extent that's an artifact of their tactical differences with the far right. Many on the far left (including anarchists, out-and-out communists or AnComs, and even some DSA types) embrace anonymity, decentralization, and _not_ documenting their efforts as core tenets of achieving their goals.
On the other hand, the far right's strategy is to beat their chest and embrace highly visible patriotic pride, which they then use to create associations between say, the flag, and their more extreme political beliefs. Then, they dog whistle and gaslight the other side about the full extent of their own and their opposition's goals as they move the goalposts towards the right to distort the mainstream view of the center.
Theoretical unknown numbers aside, I believe the latter tactical posture has been significantly more successful in the far right achieving their goals in the political mainstream. I also believe the far left's tactics have hurt them significantly in being accepted by the mainstream (and in talking to many of them privately, they often see this as a point of pride, that they're unwilling to compromise with the "normie" Democrats who e.g. sold them out throwing Bernie under the bus during Clinton's ascent to nomination). E.g. even as a small minority of the overwhelmingly peaceful BLM protests in 2020, just a few guys in black bloc throwing molotovs enabled the mainstream and far right to immediately write BLM off as a violent, communist, antifa movement that was a threat to democracy, capitalism, and the American way of life. So why throw them at all? What good did that do? (And could it have ever been enough to offset the negative PR?)
Even the slogan that many on the mainstream left embraced--defund the police--was a terrible messaging position that AFAICT almost always required immediate clarification of why that didn't mean "we want to abolish the police, all criminal statutes, and let society run wild doing whatever they want". I think it would've been a lot harder for the right to attack them if they'd have gone with something like "fund community outreach [for POC and mental health]". And then they could have even found their $$$ in the police budget later (a trick Republicans use all the time). Hell, they might have even found alignment with Republicans in the wake of mass shootings to actually address mental health on a national scale (something I'd argue is critical to our long-term well being as a country, regardless of your stance on their relation to mass shootings).
ANYWAY, I got a bit off the rails there, but far left filter bubbles--with calls for violence, personal attacks, doxxing, and all the rest of it--absolutely exist on Twitter and Reddit (among other places, I'm sure). In particular, just pop over to subs like /r/GenZedong, /r/COMPLETEANARCHY, /r/Anarchism, /r/AnarchismZ, /r/196, /r/2624, /r/JusticeReturned, or many many others. (I'd also argue that these transgressions are largely tolerated on those platforms in a way that the far-right is not, leading to an increase in demand for an increasingly fractured of ecosystem of "alternative" platforms where extreme ideas evolve and grow, rather than being diminished and ridiculed in the light. But that's a debate for another time.)
If we say that Gab isn't far-right, it's kind of like saying Grindr isn't just for gay men. That may technically be true, but in practice it's not.
Also, judging Gab by its ownership, if you look at the Wikipedia page for Gab, and go to the Antisemitism section, Gab has made multiple antisemitic statements officially. Straight from the company. The owners of the site are far-right.
Gab only exists because far-right people get banned on other platforms for breaking hate speech rules.
The far-left as a group barely exists compared to the size of the far-right. Far-left would be advocating for violence to dismantle the neoliberal system, these would be your anarchist-types.
In real life, I've never met anyone on the "far-left" when you describe it this way, while I've met plenty of openly racist and LGBT-phobic Republicans who are pro-violence: the kind of people who want to, I dunno, be a part of a coup and enter the Capitol building? The kind of people who cheer when Donald Trump tells police to rough up the suspects a little more.
What Republicans call "far-left" is just progressive center-left people nonviolently advocating for equality. They don't get banned from the generally non-political platforms because their speech isn't hateful and it isn't violent. Facebook/Twitter/TikTok happily take their money and call it a day, they don't care about politics beyond the money those politics bring in.
Platforms like Parler are much more blatant than Gab about banning liberals who join simply for existing. I guess you could say that Gab stays true to its policy against censorship, but that policy is there to benefit the far-right over anybody else, and I think that's an important detail.
And, by the way, if you head over to Gab’s homepage it’s all right-wing political stuff. None of the suggested homepage topics are non-political. It isn’t teenagers dancing or funny skits or cat videos like TikTok. I think that’s evidence showing that Gab isn’t just some impartial algorithm. It’s curated.