It's very likely your Unix-like system distribution already comes with a GIMP package. The flatpak build is very new and therefore may have shortcomings. If that is the case, ensure flatpak is installed and if clicking the link still does not prompt to install GIMP, then manually install by command line: It may not work out-of-the-box on some platforms since the flatpak technology is new. The flatpak link above should open your software installer and prompt you to install GIMP (available platforms: i386, x86-64, ARM and AArch64).
