Gnome includes Gnome Builder[2] in most distributions, which makes it incredibly easy to start making Gnome/GTK apps. Developers can also leverage Flatpak and Flathub for near-universal distribution.
0. https://www.gtk.org/docs/getting-started/hello-world
You copy paste a ten lines gtk or qt hello world, install a few packages (certainly not 20 GB), in a terminal go in the right dir, and type something like 'make'.
The one I used was from Microsoft.
Thankfully It‘s still a relatively straightforward process to create and compile an app with Xcode, which is the only option anyway. It‘s also a 30 GB download, but at least it‘s free.
I think Apple doesn't usually blunder around like Microsoft with these things, but actually pulls through long term strategies. We are in an awkward transition imho. AppKit/UIKit are super mature technologies, SwiftUI is the future but kinda lacking and Catalyst is either a shortcut to speed up Mac/iOS convergence or a backwards attempt to get more advanced iPad apps. It's all fine as long as SwiftUI is actually the future. If any of the others are relevant in 10 years, more than Obj-C is now vs Swift, I'll be disappointed.
I don't believe SwiftUI is technically 100% AppKit under the hood, but effectively is, for now.
One setting and you can make a fat jar. Double-click to run--everywhere.
One line of code and you can apply a modern UI theme that just works.