My main account is Gnart Even Close, and I am a Gnar OTP with over 500k mastery on Gnar. I am actively involved in the r/GnarMains community (I'm a mod on the subreddit, the owner of the gameplay Discord server, and an admin on the community Discord server).
Anyways, that's enough about me. I'm here to talk about the changes to Gnar. The general direction of taking power out of Mini Gnar and putting it into Mega Gnar is a good idea. The nerf to Frozen Mallet is also well deserved, and I think it will open up more itemization options for Gnar.
That being said, I don't think the implementation of the nerfs to Gnar was done well at all. Right now, Gnar's Q needs a nerf for his laning phase, not to its late game. Mini Gnar's damage already falls off late game, and all this does is make it worse while barely changing his laning phase at all. The live numbers for the CD refund are 45/50/55/60%, while the PBE numbers are 40/40/40/40%. This is a 5% nerf to levels 1-5, 10% nerf to levels 6-10, 15% to levels 11-15, and 20% from levels 16-18. The nerfs should be targeting his lane phase, with numbers like 30/40/50/60% instead. This would be a 3 second nerf at level one instead of the currently proposed 1 second. Late game, there would be no difference as opposed to the currently suggested ~1.4 second CD nerf.
The changes to Hop seem out of place to me. I guess the intent is to promote aggressive play, but that will never happen early on as long as Mini Gnar's base defensive stats are so low, the Hop CD is so long, and Gnar can simply farm and harass from range thanks to Q and his basic attacks. Meddler mentioned a nerf to Mini Gnar's early basic attack range to force him to interact with opponents more, and I think that that was a great direction. A better compensation buff would have been to revert the base HP nerf (540 -> 510) or revert the E CD nerf (18 s CD -> 22 s CD) so that he could actually afford to step up to farm rather than be zoned off and again avoid interaction by farming with Q.
Lastly, Mega Gnar does not need more damage. What Mega Gnar needs is more tools to stay alive when initiating. He has no tank steroids, and relies purely on base stats and his build to be able to survive. Mini Gnar often takes a hefty amount of damage because he has to basic attack to build rage and deal damage and is relatively short range. Giving damage reduction on Crunch would allow Mega Gnar to initiate more safely without making him oppressive. Something like 10/15/20/25/30% damage reduction for 2 seconds scaling with E rank would allow Mini Gnar to function as a tank in the late game without breaking his lane phase or removing counterplay for enemies. Considering Mega Gnar has a combined 3 seconds of CC, 2 seconds of damage reduction would not make a massive difference once he is already in the fight, but would let him more reliably live long enough to get his ult off in the first place. This also wouldn't incentivize more damage heavy builds like the bonus AD ratio buff to his ult. I personally find tankier builds to be much more enjoyable and less oppressive than the more common damage heavy builds seen lately. Gnar's kit inherently lets him take different builds, and I think a general buff to Mega Gnar's teamfighting regardless of build would be more appropriate than a buff to teamfighting for the builds that are supposed to be weak at teamfighting.
TL;DR Frozen Mallet change is good, general direction of putting power from Mini into Mega instead is good; change Q CD refund to 30/40/50/60% instead of 40%, Hop buff is weird and should be replaced with a nerf to AA range and a buff to either base HP or E CD, and the ult damage buff should be replaced with damage reduction on Crunch.