Last release in 2011.
It seems that a porting effort to GTK+3 was started, but stalled in 2015.
not sure what to do about this one. it's still in other distros: https://repology.org/project/gmpc/versions
Note that Debian are (necessary or not) applying a raft of patches:
We're not in a hurry yet.
For completeness, list of revdeps (plugins): https://qa-reports.gentoo.org/output/genrdeps/rindex/media-sound/gmpc
One existing issue bug 740156.