I have fond memories of using GitLab CI in 2018–2019 and I'm still pissed GitHub didn't just life and shift that kind of a model. Not sure about the particular issues you're running into but I remember GitLab supporting a lot of the YAML features missing in GitHub like anchors in order to build/compose stuff.

Oh and turns out GitHub also has that now: https://github.blog/changelog/2025-09-18-actions-yaml-anchor...

UPDATE: okay they botched it https://frenck.dev/github-actions-yaml-anchors-aliases-merge...