There is also `gimp3-devel` port, which uses gtk3. I have no idea if it compiles, but it is definitely worth trying at least.
i am not able to compile it, maybe problems with git lock-file. I tried to rename .git dir of gimp2, clean gimp3-devel several times, but no avail.
Code:
sudo port -v install gimp3-devel
---> Computing dependencies for gimp3-devel.
---> Fetching distfiles for gimp3-devel
Cloning into '/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_graphics_gimp3-devel/gimp3-devel/work/gimp3-devel-2.99.1-20200223'...
remote: Enumerating objects: 28320, done.
remote: Counting objects: 100% (28320/28320), done.
remote: Compressing objects: 100% (12191/12191), done.
remote: Total 28320 (delta 20389), reused 22141 (delta 16036), pack-reused 0 (from 0)
Receiving objects: 100% (28320/28320), 98.63 MiB | 1.81 MiB/s, done.
Resolving deltas: 100% (20389/20389), done.
Updating files: 100% (5081/5081), done.
fatal: unable to read tree (9046c758e7b82ffe39b1acb93160a0c9c15743ff)
Command failed: GIT_DIR=/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_graphics_gimp3-devel/gimp3-devel/work/gimp3-devel-2.99.1-20200223/.git GIT_WORK_TREE=/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_graphics_gimp3-devel/gimp3-devel/work/gimp3-devel-2.99.1-20200223 /opt/local/bin/git checkout -q 9046c758e7b82ffe39b1acb93160a0c9c15743ff 2>&1
Exit code: 128
Error: Failed to fetch gimp3-devel: Git checkout failed
Error: See /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_graphics_gimp3-devel/gimp3-devel/main.log for details.
Error: Follow https://guide.macports.org/#project.tickets if you believe there is a bug.
Error: Processing of port gimp3-devel failed