Give Gnar Ulti each time he gets Mega
On the PBE there is a Gnar buff which decreases his ult CD from 120/100/80 to 90/60/30. I think at that point it might be more intuitive to just have ult up each time you go mega, while it would not mean adding much more power as the cooldown with some cdr is very close to beeing that low anyway.