Panth is one of my go-to counters for Gnar. He's easily capable of wrecking your shit all day if you give him half a chance. As people have said, start armor, play passively, last hit as safely as you can and just count on the fact that he'll matter less and less if not fed and you, even behind in CS and Exp, will be a teamfight god with incredible lockdown potential and half-decent burst.
If you play to beat Pantheon in lane, you might well fail and get him rolling into a freakin' monster. Instead play to beat Pantheon in the overall game. I'm not saying a well-played Gnar can't wreck a Pantheon in-lane... but if you're not confident you can... don't try. Just try your best to feed him as little as possible, and if you can, keep your tower standing. He'll probably still roam using his ult to ambush your team... so ping to them that he's missing frequently and be ready to shove his tower if you see him ulting someplace else. If you're really ballsy, grab tele and follow him when he ults, but only if you're confident you can win the fight he's starting. (No assurance of that if he's been able to smack you around in lane).
But yeah... beating Panth in lane is tricky. That's where he's strongest. Beat him by not letting him use that strength until you can get to the part of the game YOU excell at... which is teamfights. The game isn't won or lost based on who got more kills in lane. It just helps.