Debugging tip:
As per: https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runnerscachegcs-section
If you encounter an error in the GitLab runner log:
gitlab gitlab-runner[7537]: ERROR: error while generating GCS pre-signed URL: asn1: structure error: tags don't match (16 vs {class:0 tag:13 length:45 isCompound:true}) {optional:false explicit:false application:false defaultValue:<nil> tag:<nil> stringType:0 timeType:0 set:false omitEmpty:false} pkcs1PrivateKey @2
Try creating a new service account in GCP and enable runner access through it to see if this helps to resolve the error.