Terminology

Project roles

The following are the project roles used by drupal.org; they do not match with the GitLab roles.
For example, an account with only the Administer maintainers and Write to VCS permissions on a project are considered co-maintainers on drupal.org side, but that account would have the Gitlab Maintainer role. This also means that only the accounts with at least the Administer maintainers and Write to VCS permissions can set the default branch, as described in Setting a default branch.

To check the roles assigned to the project members, visit  https://www.drupal.org/project/<projectname>/maintainers.json,
where projectname is the project short name (or project machine name).

Owner

The owner is the user who owns the project node. By default, it is the user who created the project, but it can be changed any time upon request.
Owners have full permissions on the project: make commits, edit the project node, add or remove co-maintainers/maintainers, change the permissions given to each co-maintainer/maintainer, maintain issues, and administer releases. They are maintainers, but differently from the other maintainers, nobody can change the permissions owners have on projects they own. The only way to change the permissions owners have is to change the project owner.

Maintainers

A maintainer has full permissions on the project: make commits, edit the project node, add or remove co-maintainers/maintainers, change the permissions given to each co-maintainer/maintainer, maintain issues, and administer releases. Differently from the owner, any user with the Administer maintainers permission, or other users like drupal.org administrators or drupal.org project moderators, can make a maintainer co-maintainer by removing one of the permissions that maintainer has.
When not differently specified, the owner is included in the list of maintainers.

Co-maintainers

A co-maintainer does not have all the permissions on the project. Usually, a co-maintainer can commit code. Most often, co-maintainers are given all the permissions, except the permission to administer maintainers/co-maintainers; the exact permissions each co-maintainer has are assigned by somebody with the Administer maintainers permission on that specific project.