Gitlab-ce / omnibus / GitlabCI: pipeline stuck in pending state

I have a gitlab omnibus instance (13.0.6) installed on a VPS and runner (same gitlab version) installed on compute engine on google cloud platform.
The project is correctly configured and runner (specific) is correctly connected (the green dot is visible in project settings).
When pipeline is triggered after a push, it will be indefinitely stuck in pending state!!

is there a way to see log in runner machine? so I can see what’s going wrong!

When I execute on runner machine (gcp): gitlab-runner run, the job is executed (I can see it in gitlab) and it finishes successfully.
What am I missing? I already configured runners for another projects and in different environment (outside of GCP) and it works like a charm.
I failed to find gitlab-runner log file. Where can I find them?

Thanks for taking the time to be thorough in your request, it really helps! :blush: