There is a new wayland extension, that allows for rendering at fractional scales directly in the application, so when you use a display with fractional scaling, there is no downscaling done by the compositor.

Support for this kind of rendering is going to wait until Gtk 5 -- implementing it in Gtk 4 would break ABI, and then the world would have to listen to cries about Yet Another Breakage. The current system, rendering at integer scale and then compositor-downscale is working fine since Gtk 3. Btw, exactly this way is how Apple does it, and it was lauded as a great way to support scaling.