If there is no link to a checklist, you will need to create and document the learning path as you go. The links to documentation are just to help you get started with that process. Take a look at the Geo Checklist for how to get started and put a (WIP) after your link to let people know they won't get the expert badge by finishing the list in its current state.
Only work on one subject at a time. Inform your manager which one you will be working on, so that they can let you know if there is a different area where the team really needs expertise. Always try to answer tickets on other advanced topics, but when it is time to do some dedicated learning, focus on one area.
These are in the rough order of what our team needs, but check with your manager, and we need everyone to know LDAP so please do that first.
GitLab CI (WIP)
GitLab API Checklist (WIP)