Installing it through Flatpak here is actually fewer commands than your apt example.

You just run this to add the repository, > flatpak remote-add --user flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo

Then this to install it > flatpak install --user flathub-beta org.gimp.GIMP

The next two commands let you switch between the beta and stable branches, which is something apt can't do.