1) Very, very operationally-simple deployments are available. It can run with just SQLite, no external services (e.g. database daemons, queuing systems, all the other stuff that GitLab likes to have) whatsoever.
2) It looks and feels more like Github, which is nice if you prefer that UI.
3) The site's far lighter-weight and snappier than GL, when you're using it.
4) You can (probably—workloads vary) serve 1-100 users with the dumbest possible deployment, running on a potato. It's far more resource-efficient than GL—which, it's not hard to be more resource efficient than GL, since it's an absolute beast, but Gitea comes in way under it.
Downsides:
1) Missing some stuff GL offers (like integrated CI)
2) Lacks some features that make serving at enormous scales (of the sort you almost certainly won't hit unless providing a Github/Gitlab-like service to the world) impractical.
3) Fewer just-works 3rd party integrations, especially commercial ones.
