https://en.wikipedia.org/wiki/GIMP#History
The application subsequently formed part of the GNU software collection
And it's part of the list of "GNU Software" on this page: https://www.gnu.org/software/software.html (And no, that's not a list of all GPL-licensed software, it's a list of GNU software).You're correct that it's not "maintained directly by the FSF", that's not how GNU operates, but I feel that GNU and the FSF has a responsibility for the health of the projects which are part of the GNU project.