Hi Daniel,
I recently helped implement google auth using a service account to access a google API, if I remember correctly you should use the CreateSignedAsymmetricTokenWithPemKey action (JWT module) to create the token and when generating the keys google returns you a json file with the private key were each part of the private key is separated by \n, you cannot just copy that field, it should go from this:
"-----BEGIN PRIVATE KEY-----\nABCD\nABCD\n[...]\n-----END PRIVATE KEY-----\n"
To this:
"-----BEGIN PRIVATE KEY-----
ABCD
ABCD
[...]
-----END PRIVATE KEY-----"
Feed the private key string like that to the action and don't forget to fill in the rest of the mandatory fields like KeyId, Issuer, IssuedAt, Audience and Claims.
Hope it helps.