1) Really good at doing something. Up to the point of being gerkinly good.
2) A bit of a nobhead in any kind of way.
(Yes, these two conflicting opinions do make it extremely hard to tell what is meant sometimes. It's open to the receiver's interpretation to prevent unwanted offense and tears.)
3) A particularly nice gerkin.
Will: I'm pretty solid at chess.
Emily: You're not just solid, you're a Murkin' Gerkin
Lydia: Hey John! That new shampoo you're using is making your hair smell like wet dog.
John: Stop being a Murkin' Gerkin.
Bethany: Mmmmmmmmmmm... that was one hell of a Murkin' Gerkin.