It's not real clear, which is a shame because the development story is what makes me so excited about it. Basically you want to look at the gnome developer docu

It's not real clear, which is a shame because the development story is what makes me so excited about it. Basically you want to look at the gnome developer documentation (https://developer.gnome.org/ https://developer.gnome.org/) this is also often incomplete or outdated but it's a start. A standard gtk app will probably run fine on the phone, maybe with some minor tweaking. For compiling, linking, etc you just use the standard c tools, or the python ones. I can't think of the name right now but there is a widget gallery installed with the libgtk-3-dev (I think) package that includes a heap of code samples. There's also this whole gui xml designer thing they keep trying to push, but the best thing about librem development is that you can start with a single c file and expand from there.