Hasn't Gnar basically been garbage for years now? Ever since he got nerfed after his original reign of terror, he's been by and large forgotten.
Buffing Mega Gnar's resistances won't really help the fact that a lot of the current top lane picks can destroy Mini Gnar easily (Mini Gnar's mostly good at kiting and playing keep-away, and even then most of his lane opponents don't have much trouble forcing him out of lane due to his low HP5 and lack of sustain).
He's barely played anymore and as far as I can tell has been dead last in win-rate for top lane for some time.
Just glancing at champion.gg, the only top laners with lower win-rates than Gnar currently are Trundle, Aatrox, and Akali. Trundle's being buffed too, and the latter two just got dumpstered by nerfs. And even then, the dumpstered Aatrox's win-rate is only just barely lower than Gnar's.