His range is a joke early on. Honestly, I play Gnar a lot so I know what I'm talking about.
The Q can be blocked by minions since it turns around once it hits one. Also it's kinda THE ONLY THING HE CAN DO WITH RANGE. Mini Gnar has only two active abilties, and most players will save the jump for disengaging. On top of that the jump range isn't even really far.
On top of that he can't even use his limited range to harass instantly - or he goes into mega Gnar with Zero range.
A fulltank Gnar is basically worthless the moment he misses his cc when going to mega Gnar... His auto-attack passive won't help either since a straight tank build doesn't come with much attack speed. Real fulltanks like nautilus are more scarier than this champ when it comes to cc and tankyness.
"Can initiate on a whim"
Lol. LOL. Are you serious? You do realize that most of Gnar's power budget lies in his tranformation? Especially tranforming while a E hop, wich is the engage with the most range. You can even SEE his rage bar. Poke him until he goes red, back of for like five seconds so he can't use the jump to enage on you and BAM , you're clean and you can feel free to farm without a ranged champ harassing you.
Also: stay away from walls when Mega Gnar is incoming. The thing people I play against never do, so well, their fault for putting themselves into the perfect spot for getting squashed into the wall for bonus damage.
Tl, dr: Fuck up his rage management. Know when he transforms. And don't start auto attack duels midgame with Mini gnar unless you surely know you win. That's all there is to it. Also jungle help is always good since Gnar jump's not really far.