I try to get involved using GitLab in combination with Salesforce. However, the trailhead module 'Build an Automated CI/CD Pipeline with GitLab' seems to have been taken offline, and some other resource links do not function either. However, I still can login using my Salesforce account. Is there any up to date GitLab - Salesforce documents available to get into the topic? How is the current opinion on the topic?
Seems the specific trailhead you linked has been retired. In terms of Gitlab/Salesforce documentation, it seems the following Salesforce blog post related to when that trailhead was created contains many resources
- SFDX project template for GitLab where the README has a guide to get started with GitLab and Salesforce development.
- sfdx-cicd-template is a project that contains a fully configured CI pipeline following the package development module.
- SFDX gitlab-ci.yml template along with other templates on Gitlab
There's also this video walkthrough (has very low audio) that goes over setting it up as well.