Gimp is also possibly designed only for 'make' and never supposed to work in any kind of IDE.
Well, there is not a project file or anything. That kind of thinking is pretty Windows-centric, although some Linux apps facilitate it (KDevelop).
People are gradually moving away from make/autotools to things like meson, scons, etc.., but it has been the standard for a long time on Linux, and GIMP still uses it.
The reason I mention Linux is because opensource software is, in the overwhelming majority, developed on Linux. That's where the best development infrastructure is -> most volunteer devs do not want to bother developing for Windows.
(not coincidentally, building GIMP for Linux is easy. Make sure GEGL and BABL installs are up to date, then run the standard './autogen.sh;./configure;make;sudo make install')
.. why do you think GIMP is a pixel art program, though? It's more like Photoshop -- an image editor that you *can* use to do pixeling.
It does make me wonder if you are trolling... have you even used GIMP?