That depends. What are you wanting to accomplish more broadly with the integration?
I'll mention a couple things that might be relevant - you could have the git repo reference a branch or an immutable namespace hash on Unison Share. And as part of your git repo's CI, pull the Unison code and compile and/or deploy it or whatever you need to do.
There's support for webhooks on Unison Share as well, so you can do things like "open a PR to bump the dependency on the git repo whenever a new commit is pushed to branch XYZ on Unison Share".
Basically, with webhooks on GH and/or Unison Share and a bit of scripting you can set up whatever workflow you want.
Feel free to come by the Discord https://unison-lang.org/discord if you're wanting to try out Unison but not sure how best to integrate with an existing git repo.
It's open source, you can create a free account with GitHub OAuth, and you can push projects there and collaborate on them, open PRs, publish releases, etc. It's very quick to pick up if you're already familiar with GitHub.
The tool you use to interact with the code database keeps track of the changes in an append-only log - if you're familiar with git, the commands for tracking changes echo those of git (push, pull, merge, etc) and many of them integrate with git tooling.