Guard Posts are never attacked by Dyson Gatlings, though I've often wished they would. Also, Hostile gatlings are meant to shot both AI and Player units, so no surprise there.
I agree its lame that your gatlings are apparently idling away rather than going anywhere, it's a bug that has been in the game before but I think it was fixed at some point. They might be waiting for a critical mass of gatlings before moving out, but I'm not a 100% on that. You don't need a friendly planet but be aware that they don't range very far from their home, 1-3 hops at most last time I knew for sure.