google cloud platform – gitea action authentication with gcloud fails

can not connect gitea runner to GCP.
I have a gitea runner and gitea server running as dockers on-premise.
I would like to use the gitea runner to deploy code into GCP.
When i run the command

- name: Authenticate with Google Cloud
        id: auth
        uses: 'google-github-actions/auth@v0'
        with:
          credentials_json: '{{ $secrets.NOT_A_SECRET }} # (the real secret contains the content of a key json file of google service account)

I get the error :

google-github-actions/auth failed with: retry function failed after 1 attempt: failed to parse service account key JSON credentials: unexpected token { in JSON at position 1