You can use most of the popular GUI libs, there's a couple Lisp-specific ones but they're not nearly as featureful. https://lispcookbook.github.io/cl-cookbook/gui.html collects most of them; though for gtk4 I'd recommend https://github.com/crategus/cl-cffi-gtk4/ But no matter what you use I think the experience is going to be a bit rough. CLOG is probably the most interesting non-traditional framework but you do have to accept some web idioms.

I'm confused by the GP's assertion that backends aren't a good fit because of the threading model, to me they're one of the best fits. The 10k problem isn't a concern for most software, and in any case there are ways around it. (I don't know what Google Flights does but even at their scale they haven't moved off SBCL. And Hacker News itself runs fine on SBCL, though there probably aren't 10k concurrent connections.) In the ecosystem, there's the https://github.com/fukamachi/woo webserver which binds to libev to handle similar scale as Go was advertising about 10 years ago. And besides some work going on recently on the SBCL dev mailing list to add native fibers/green-threads, there's been non-native versions of them before, and basically any concurrency model you can think of has been built on top of SBCL by someone. (STM, actors, async (one even built on libuv), channels, promises/futures...)