gpo does not seem to have a delete command.
Also, I wonder if instead of specifying deletion by how many days old any may be, if during subscription for a particular podcast it could be set for how many days to typically keep before deletion (or marked for deletion). Some podcasts of short time-relevance such as newscasts don't need to be kept more than a day or so; some like long weekly shows would be nice to keep for 6 days or so until you have a chance to listen.