As of August 2025, Zed had 150K monthly active users. That was before it supported Windows; the number is much higher now (although not publicly reported).
I'd be very surprised to learn that any other Rust UI crate has more real-world usage than GPUI!
users is a finicky metric. When Palia came out, a game you most likely have never heard of, I wrote a desktop installer for it with Druid, which a few million people downloaded and used to install and run Palia. Only a handful of people worked on this codebase, maybe three or four while I was there, but principally me and one other engineer.
The more salient metrics would be things like how many people know how to use the framework, the variety of use-cases its good for solving, how easy it is to hire or get help with it, etc. As for Druid, Druid is already officially unmaintained, its core developer having moved on to work on Xilem instead. (my experience, for the record, was positive, I very much enjoyed working with Druid.)
I don't know that it's all that meaningful to discuss the component library as if it were its own UI framework. None of the other rust UI frameworks have distinct component libraries with distinct usage data either
gpui itself is spun out of the zed editor, so I'd say it probably has more real-world usage than the majority of rust UI crates