good question! there was/is, but it uses an internal-only API feature – [the `stream` collection for `Activities.list`](https://developers.google.com/+/api/latest/activities/list#collection) – so it was never fully public. (i wrote it while i was at google.)
if that feature ever goes public, it’ll be trivial to support in [activitystreams-unofficial](https://github.com/snarfed/activitystreams-unofficial) by [changing `’public’` to `group_id` here and passing `’stream’`](https://github.com/snarfed/activitystreams-unofficial/blob/master/googleplus.py#L98). i’m not holding my breath, though.
of course, that’s for your stream, ie posts from the people in your circles. if you just want a single person’s feed, there are [lots of services and libraries that do that today](https://www.google.com/search?q=google%2B+rss+feed), including activitystreams-unofficial.