The example to point to is Blender, for inspiration on how to manage this successfully. Ton has done a heck of a job making Blender into a real alternative, even with the codebase existing and being worked on since 1995. The UX and UI overhaul in 2018 (Blender 2.8) was a big undertaking, that had a big impact on the success of Blender, maybe something similar would be possible for Gimp too.

Granted, Ton figured out a project of that scale needed funding, but overall, Blender is a huge success as a FOSS tool. Maybe the Gimp and Blender developers so sit down together and have a chat, the tools are often used together after all.