The Gimp Downloads page says run brew install gimp to install Gimp on a mac. Doesn't work. This does.
brew install gimp
Oddly, downloading from the download page yielded (for me) an unsigned version of the app. Installing via brew installed a signed version.
$ brew cask install gimp
Right, searching for available packages that brew (homebrew) has. I'm surprised I didn't just try this without looking for it in the first place.
$ brew search