I'm not aware of any spring gate closers for the tubular pipe. However, I'd envisage that standard gate springs would work in a similar fashion. You don't have a flat surface to screw into, but that shouldn't prevent you from Tek-screwing the mount to your tubular post. The Tek-screw should be strong enough, but you could place a bolt right through the tube for extra strength.
My logic might be flawed, but the return strength of the spring is governed by it being pulled out of alignment. I suspect a similar effect will happen whether the surface is circular or flat. The effectiveness of my proposed use would likely be determined by the type of hinge on the gate. Are they standard hinges that have been welded to the gate (like on an internal door), or are they specific pipe clamp hinges?