I just did a few waves of this, to start it off you get status from the Militia Outrider to be able to participate. There doesn't seem to be any rank/experience points gain of any kind, but at the end of a wave a gold chest appears for everyone who participated. All I've seen form these chests are deaspected crystals of a random amount. Other than those it seems theres a rare chance for gear to drop from the Garleans. I got a Sheepskin Harness (Grey) +3 and a Cotton Cowl +3 myself.