Running gr_modtool results in a mystifying Python error:
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe8 in position 5:
invalid continuation byte

That is a Debian packaging of gnuradio bug that mint inherited.
This is also found in Debian 10 "Buster" gnuradio 3.7.13.4-4.
And likely also in recent Ubuntu releases.

The problem comes from Python byte-compiled files used as templates.

The solution is to remove all .pyc files found under
/usr/share/gnuradio/modtool/gr-newmod
especially:
/usr/share/gnuradio/modtool/gr-newmod/python/build_utils_codes.pyc
/usr/share/gnuradio/modtool/gr-newmod/python/build_utils.pyc
/usr/share/gnuradio/modtool/gr-newmod/python/__init__.pyc

Recent (gnuradio 3.8) Debian packages no longer byte-compile any Python
under /usr/share/gnuradio/ - solving this problem.

Easiest end-user quick fix is just
 `sudo rm /usr/share/gnuradio/modtool/gr-newmod/python/*pyc`

Correct packaging fix is to add a debian/gnuradio.bcep file containing
dir|-4.0|/usr/share/gnuradio/|.*

to keep the .pyc files out of the template and example directories.

Thanks César for reporting this,
-Maitland