That's not really so obvious. You'd be surprised at the number of anti-gun advocates that either own guns themselves or employ armed security if they are wealthy enough to do so.
Sure, such people exist. It's not true that anti-gun liberals have literally no guns - there's plenty of hypocrisy to go around. But there are relatively few people on the leftward end of the spectrum who have both the means and the willingness to stage an armed rebellion of any real consequence - the left is armed disproportionately poorly relative to the right, and has not generally made any real preparations for an insurgency. I'll grant that a few tiny extremist groups of Antifa or New Black Panthers or whatever could choose to launch terrorist attacks, but we're not talking about very many people here. We're not going to get any more left-wing violence than we had in the late 1960s and early 1970s with the Weather Underground, SLA, original Black Panthers, etc.
If you lay out a plausible scenario where we get from left-wing groups being unhappy about their election results to the point where there are armed left-wing insurgencies in significant parts of the country, I'm willing to take it a bit more seriously. Right now, though, that seems far-fetched.
OTOH, I think there really is a significant chance of armed right-wing insurgency in the event the Democrats regain political power. On the right, there are a lot more people who have significant stashes of weapons all the way up to small private armories, along with shelters containing provisions, small armed militias, and the kind of rhetoric that more credibly signals a willingness to fight. There is also a substantially more powerful series of propaganda channels reaching a lot more like-minded people. Finally, there's a strong concern that they are losing power permanently for demographic reasons - losing the election isn't just about this one election, but about their prospects in the future as well.
I was watching Alex Jones and Breitbart pretty closely in the lead-up to the 2016 election because I was concerned that a narrow Clinton victory would result in the sort of situation that can blow up into a real insurgency. Once you have a whole lot of armed people who believe in conspiracy theories for why the government is illegitimate, and the one person they hate the most gets narrowly elected president amid numerous claims of fraud that get repeated ad nauseam on all their media outlets, and the candidate they support is fanning the flames in every possible way, things can get pretty dangerous.
It's not too hard to see how some spark could snowball rapidly into large-scale insurgency by loosely affiliated groups in rural parts of the Mountain West, Appalachia, and the Deep South. Perhaps a bigger takeover of federal lands by an armed militia is responded to heavy-handedly in the early days of Clinton II. A Waco-style event happens, which serves as a casus belli for further rebellion. A few National Guardmen and military personnel defect and join in; most don't, but aren't too keen on suppressing it either. Jones and Limbaugh and all the rest provide vocal support over the airwaves and internet, and attempts to suppress them simply make them even more popular. Pretty soon there are armed takeovers, IEDs, suicide bombers, ambushes, and whatnot across a large swath of the country.
So that's the sort of way a right-wing insurgency would start. But it's not at all clear how a left-wing one would.