I personally use gitea but there is also a community version of gitlab that has way more power than I need.
Gitea can import a repo from GitHub but I don’t know whether it can also push updates out as one never tried to do that.
I picked gitea as I didn’t need all of the extra power of gitlab and they were the first two options I found. I don’t deploy it using portainer but all of my stacks are set up as git repos in portainer and using the webhook feature it’ll auto pull and redeploy whenever I push to it