graphics/gimp has Python support ON by default. This pulls in graphics/py-gimp which is a rather old port and requires Python 2.7 explicitly. From the Makefile for graphics/py-gimp:
DEPRECATED= Uses deprecated version of python
The Fedora community has a similar problem, just before their 32 release: https://bugzilla.redhat.com/show_bug.cgi?id=1737933
GIMP3 is in the making and it *should* default to Python3, however no info is available on the official Roadmap: https://wiki.gimp.org/wiki/Roadmap#GIMP_3.0
I can start tinkering with GIMP3 to see whether it's possible to re-use the current Makefile, but that would still provide a "gimp-devel" port at best.
The alternative would be to disable Python support in graphics/gimp so that the port continues building past its expiration date.