Captain America W/GL Ring Vs Thor

Cap can use the ring to it's fullest. He has training.

Does he retain his shield?

Sure.

Given Steve's willpower coupled with actual training with the ring and the fact he'd still keep his shield, its certainly possible he'd be able to beat Thor. Still, Mjolnir is a potent weapon to have and ultimately trumps power rings due to its energy manipulating and absorbing capabilities.