This is currently limited to secrets from your current project.
You can get secrets from different projects via

gcloud secrets versions access projects/<ID OR NUMBER>/secrets/<NAME>/versions/latest --secret <NAME>

I think it would be nice if the input secret name could be parsed first, to see if it is a fully qualified name. If so, the name should be extracted and the command from above should be used (maybe with a shortcut to avoid the need for /versions/latest).