Assuming you meant "GNOME" Builder rather than GtkBuilder (the template loader for GTK), I'm the author of it and can help figure out what is going wrong on your system wrt not compiling anything.
Just file an issue on gitlab and I'll take a look.
Usually when we come across bugs like this it's related to distro packaging as we have no hand in that and the Flatpak packages I maintain generally do-as-advertised.
I don't know what's wrong exactly, I think it's Flatpak related. From what I remember, it had something to do with a certain version of the GNOME SDK not being installed (I think it was looking for a version of something that didn't exist?) so it failed a dependency check when I hit the install button from the Builder popup. I think this caused the tool to get into a faulty state where it thought the dependency was installed but simultaneously suggested installing the dependency, so it freaked out. I'm usually working on Ubuntu so combining the outdated Ubuntu stable-ish packages with Flatpak's bleeding edge stuff causes issues from time to time.
This could've been a temporary problem and I could've had a streak of bad luck, or maybe my deskop and laptop are somehow just set up plain wrong. I'm no expert in Flatpak internals. I also don't have Builder set up any more so reproducing the problem is kind of hard. Next time I'm giving it a try, I'll try to keep in mind to actually post a bug report.
42 will be out in a couple weeks, and it has a lot of work to make flatpak stuff work better in the presence of "system" installations (rather than user installations where we normally install SDKs).
Hope you'll try it out! (It's also available on Nightly).
Is GNOME Builder intended to be usable as a general-purpose IDE, beyond just developing GNOME applications? It seems like with a bit of work it could be a very nice alternative to the usual picks.
There are really only a few of us that make Builder, mostly just in our free time. So that means we have to be very focused with our time and choose to focus on making it easier to write GNOME software.
That said, the whole thing is an open ended plugin architecture and we are very liberal about accepting all sorts of things. Many people do use it successfully outside it’s intended scope.
Just file an issue on gitlab and I'll take a look.
Usually when we come across bugs like this it's related to distro packaging as we have no hand in that and the Flatpak packages I maintain generally do-as-advertised.