It looks like this project is dead and unmaintained.
On my Debian system I have Ruby 3.1 and I can't use this project.

I suggest to use one of its forks. See https://github.com/tomgi/git_stats/forks

For example I installed nova_git_stats on Debian and it works like a charm.
https://github.com/mishina2228/nova_git_stats
https://rubygems.org/gems/nova_git_stats