Obligatory check : are you sure you really need a forge? (that’s the name we use to designate tools like Github/Gitlab/Gitea/etc). You can do a lot with git alone : you can host repositories on your server, clone them through ssh (or even http with git http-backend, although it requires a bit of setup), push, pull, create branches, create notes, etc. And the best of it : you can even have CI/CD scripts as post-receive hooks that will run your tests, deploy your app, or reject the changes if something is not right.

The only thing you have to do is to create the repos on your server with the --bare flag, as in git init --bare, this will create a repos that is basically only what you usually have in the .git directory, and will avoid having errors because you pushed to a branch that is not the currently one checked. It will also keep the repos clean, without artifacts (provided you run your build tasks elsewhere, obviously), so it will make all your sources really easy to backup.

And to discuss issues and changes, there is always email. :) There is also this, a code review tool that just pop up on HN.

And it works with Github! :) Just add a git remote to Github, and you can push to it or fetch from it. You can even setup hooks to sync with it. I publish my FOSS projects both on Github and Gitlab, and the only thing I do to propagate changes is to push to my local bare repos that I use for easy backups, they each have a post-update hook which propagates the change everywhere it needs to be (on Github, Gitlab, various machines in my local network, which then have their own post-update hooks to deploy the app/lib). The final touch to that : having this ~/git/ directory that contains all my bare repos (which are only a few hundred MB so fit perfectly in my backups) allowed me to create a git_grep_all script to do code search in all my repos at once (who needs elasticsearch anyway :D ) :

#!/usr/bin/env bash
# grep recursively bare repos

INITIAL_DIR=$(pwd)
for dir in $(find . -name HEAD -exec dirname '{}' \;); do
  pushd $dir > /dev/null
  git grep "$*" HEAD > /dev/null
  if [[ "$?" = "0" ]]; then
    pwd
    git grep "$*" HEAD
    echo
  fi

  popd > /dev/null
done

(note that it uses pushd and popd, which are bash builtins, other shells should use other ways to change directories)

The reason why you may still want a forge is if you have non tech people who should be able to work on issues/epics/documentation/etc.

Have a look at Forgejo which is a soft fork run by a nonprofit organization of Gitea which is owned by a for-profit company.

https://forgejo.org/

It need very little system resources and still gives you all the common features you know from commercial Git hosting providers.

And yes, you can mirror existing Git repos using a web UI.

Why do you want that? Plain git can do everything you actually need.