I can't speak for GPUI specifically, but in general for Rust UI toolkits I'm seeing about 10-15mb for an O3 build with LTO enabled and when including things like ICU data, a WGPU-based renderer, SVG rendering support, and a "full fat" async HTTP client. More minimal builds with features disabled and/or Os/Oz can bring that down to more like 5mb.

(obviously you can also take things much higher by building more functionality into your application, but that gives you an idea of the kind of "base size" achievable).