How to decrypt Terraform states stored in GitLab backend
Assuming you are using the GitLab Terraform state feature in your self managed instance and you are using the embded backup utility provided by GitLab.
The Terraform state files are encrypted before they are stored. This means that you cannot retreiv the content at rest. For this purpose, GitLab use application secrets (and derive new secrets from thoses keys when needed) to encrypt sensitive content.
You want to retreiv the content of a state file from a GitLab backup. Like explained in this issue, it's not possible to easily retreiv a decrypted content is the instance is offline.