You should totally switch to GNU/Linux, though.
The most popular libre graphics tools will run like a normal Mac app, yes. If the one you are interested in does not, it might run with 'X11,' a unix compatibility tool; and you can run a GNU+Linux system on your Mac desktop with the libre Virtual Box tool, which is convenient but may be slow. If you need speed, you can install GNU+Linux directly and reboot into it when you need to.
If you got a written note from Steve.
Ask Apple CEOs & stockholders if they will let loose the leash on engineers to bring Apple back to the reason why it was founded. THOUGH THIS WILL NEVER HAPPEN!
Many FOSS tools also run on OS X. Ubuntu will also run on mac hardware.