Forgejo is my go to, I ran it in a GCP micro instance, which has 768 MB ram and a piddling processor. One of my friends works for a company that had all their devs run a local instance in addition to the main repo, it was that light.

Gitea is the former go to, but gitea was hijacked and stolen from the community by a for profit company. Forgejo is currently a drop in replacement fork, but with added privacy features, future federation options, and a reputable parent organization.