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They're only advertising to one peer. It's possible they have other peering arrangements in place they're not advertising to. Or maybe they're single homed. They also only have a single IPv4 /24 from ARIN. No IPv6. Not terribly robust or impressive. I've never used Gab or looked at their infrastructure before. A quick look shows they're behind Cloudflare. No clue where they're actually hosted or what the infra looks…
You're right that the single peer is a concern. Maybe they have others that are hidden for now, like you said? I don't think they need more than a few IP addresses since most of their hardware could sit on a private network with only a single load balancer visible from the Internet. Also, a small ASN doesn't really need IPv6 compatibility to function, and won't for many years to come. Since they're still using Cloudf…
They could be doing anycast, but that's pretty sophisticated. They could also only advertise the /24 from one DC at a time, but that's error prone. Or maybe advertise from both and forward to the live DC over a private link/VPN.
I do see in Octoberish, they were peering with a second network, and they have second network in their IRR records. Could just be so they can use one of the DDoS filter companies that advertises your netblock, and sends you clean traffic. Can't do that (easily) if you get IPs from your transit provider and it's not a service they support.