The package needs porting to Python 3.13.
Created attachment 923475 [details] gramps-6.0.0.ebuild
I am not in a position to act as proxy-maintainer, but I can report that a lightly-edited ebuild bumping the revision to 6.0.0, adding dev-python/orjson to RDEPEND and building with PYTHON_SINGLE_TARGET python3_13 succeeds. I cannot test exhaustively but simple functionality works as expected.