The error you are getting is from your Kubernetes cluster being unable to pull container image from GitLab.
Providing a temporary credential to pull the containers as deploy time is not a great solution. For example, say your Kubernetes cluster needs to scale up by adding another node, the node will come up and try and pull the container, but the secret will ...
If you could let us know what helm chart you have used to install prom-operator, that might help refine the answer.
But in case of kube-prometheus-stack, in the values.yaml of the , use the additionalScrapeConfigs section to describe jobs that are external to k8s system and re-deploy.
Or use serviceMonitors as described in this article