Garside has scored 36 tries in 54 appearances since joining Sheffield from York at the start of 2013.
Eagles coach Mark Aston said: “We’re sorry to see him go because over the last two years he has started to develop into a very good player.
“It’s always sad to see someone leave but he goes with our best wishes. He’s going full-time which is the reason he has left; we’re not in a position to offer him that at the moment.
“He’s been a good performer for us and has transformed himself from a centre to a back-rower with try scoring ability. He has been a pleasure to coach and now we will look to bring in a suitable replacement.”