You are looking at the wrong number :) That's the total amount of forks on GitHub. Whereas only 15 of them had any commits by people who forked the repository. Among those 15 people:
- One is a regular GIMP contributor.
- One couldn't wait another day for a macOS build of version 2.10.12.
- One patched file association on Windows and submitted his patch to the upstream project.
- Some of the others worked on their pet peeves, and some of them discussed it on the upstream bug tracker.
You might want revisiting your definition of a fork :)
None of which really compete, but yeah, literally hundreds. Just one big one though, GIMP Painter.