I sadly have no clue how to use the files you put on salsa to build my
own package. I am merely a Debian user, not a developer or maintainer,
but I will look into it how to create a .deb package from those files
(probably dpkg-buildpackage?). If I succeed, I can give you more feedback.
I'll give a (very quick) guide on how to build it.
First you need the basic build utilities for building a .deb: build-essential debhelper devscripts
Then clone https://salsa.debian.org/science-team/geant4 (tip: do this in an empty folder already for the output)
To build the .deb, you simply run `debuild` (better in this case: `debuild --no-lintian --no-sign`).
The final .deb is in the parent folder of the sources.
Building might fail if you have build dependencies missing, they are listed in debian/control.
Note that dh-sequence-python3 is provided by dh-python.