Heartsteel
The usual Heartsteel is great, and I think if you don't die early you can beat him if he messes up with his jump!
Iceborn Gauntlet
I think Iceborn is probably really good here. I haven't tried it really yet, but would like to. It probably helps with survivability and chasing him down.
Plated Steelcaps
Tabi is probably good against Gnar when behind or even in general, but I like swifties and usually stick with that. Perhaps Tabi is theoretically the move though? Not sure.