I've seen and handled a Gitzo "0" series, but it was a few years ago now. Not use now exactly which model.
My impression was that it was a marvel in engineering, in that it was built to the same standards as your Series 3, only smaller. And I am sure that is why it is so relatively expensive.
I figured it would do as good a job as any 20mm legs could do. And that was before the big emphasis now on high end mirror-less cameras. My sense was that it was a strange solution in search of a problem but it would make for a very nice table top tripod with perhaps only one additional section extended. And that would be for a modest DSLR.
I thought one of the nice features is that it is only a 3 section, where many others try to be all things to all people and go to 4 or 5 sections to arrive at a max height spec that is unusable due to the instability of those necessarily tiny lower sections. I can't say I would ever use the GT-531 fully extended, even with a mirrorless camera.
I think it is a marvel because most other "table top" tripods I've seen are built to a price point in the opposite extreme. And that makes for a bad combination.
Kirk makes a "Mighty Low Boy", which is basically a cut-down "Manfrotto 190" (actually the predecessor). RRS came out with their own version of that, but at a higher price point. I think RRS no longer makes their version?
I never really understood why I would want one of those, rather than use a Gitzo Series 2, or something similar. I would really need the more compact and perhaps lighter weight. The main advantage (for me) of an ~12" long folded length is the ability to shoot low without a lot of leg spread out, which is often a problem on a real table top (unless it is a rather large table).
I'm seeing the same thing you - this is a rather niche market. Both RRS and Kirk have since introduced true tiny tabletop models, obviously intended for mirror-less cameras.
If I were going for the ultimate tabletop I guess I would bite the bullet and get the Gitzo. Only because then I know what I would be getting and know that that very difficult engineering problem was implemented with quality parts. But it is a market I don't search very often because I've never had a pressing need for that. When I want small I just use my old Gitzo Series 2.
Wistful thinking... many years ago, around when the G-Lock/6X tripods were introduced, someone blew out a bunch of the older Series 1's, probably G1227 or G1228, for $75 a pop. Or maybe it was the "0" series - it's been many years now. I always regretted not going for that, even though I had no pressing need. Any Gitzo CF for under $100... hehe.