I've managed to solve this. I've added secret to gocd cluster and using config XML, it is connected to gocd-agent pod.
Image used for gocd-agent has gcloud installed.
Afterwards, there is pipeline that is dependent on built artifact and uses it to copy archive to bucket on same GCE project where kubernetes cluster is.
Service account is used by gcloud command to authenticate and afterwards, simple gsutil copy command just transfers archive to bucket.
If there are questions, please let me know.
Kind regards,
B