He was. Briefly. I'm not sure if there are plans to bring him back, but his character left about 3/4 of the way through this last season. He was good on the show, but nothing to write home about, IMO.
I've never seen one minute of Smallville, but I've heard people talk about JH having played GA. When he showed up on Revenge, I remember thinking "he's cute, but he's no Stephen Amell". Although, admittedly, I'm quite possibly SA's #1 fan, so... ;)