If you mean you want to make it so your WordPress site is on GitHub/GitLab/etc. it's fairly simple:

• Make sure you have SSH keys configured on your server.
• Copy your public SSH key on the server
• On the Git service you've decided to go with, add the public key. I'm going to assume GitHub going forward
• for github, you'd go here: https://github.com/settings/keys
• Create the repository on the git service of your choice (I use GitHub -- you pay $7.00USD a month, and you get 5 private repositories)
• Once you create the repo, go to the repo. It will have the commands you need to run to set up your existing code as a repository.
• You'll want to back up your database and upload it as well, so follow this guide to use mysqldump

Hi - Straightforward, but is this the most secure way? The one-click distros give you 1 user, root, and following these steps would add root's public key to github/bitbucket, right? And any git-pull would pull down files with root:root permissions, no?

If this is not correct, please include a blurb about how to add another non-root user as part of the steps. Or provide info about chown -R www-data:www-data after a pull as root to ensure apache (or ngynx) won't have trouble getting at them.