No one considers Gnar as a high-mechanic champion though and in my opinion, he is harder than Riven or Yasuo and I'll explain why. He has to fill A LOT of purposes in a game. In the early game, where he stays in lane, he is mostly a ranged champion, since he doesn't get MEGAGNAR often. Therefore, you need ADC mechanics, where you poke with Autos and your Q, and you have an escape on E and a 3-hit proc on W. That said, your AA range is only 400 (getting to 500 at level 18), which is the lowest of all ranged champions, making AA poking harder than all of them, thus requiring great positioning and decision making to avoid minion aggro and retaliation. Also, he doesn't have resistances and hp regen early, so avoiding all damage is essential.
Then, it's the bar management. Knowing when to engage depending on the stage of the game and the situation you are in is highly dependant on your rage bar. It's better to be Mini when 1v1ing, especially after level 9, but have to be Mega when a teamfight breaks out. All these need careful management.
Also, as Megagnar, you need to also do a lot of things. A good engage is the most critical, if you fail that, you are useless as Mega. Then, you have to follow up on the damage with your abilties, inserting an AA inbetween for maximum damage efficient, and all that is hard in the cluster that is going on. And then, once you turn back to Mini, you are still the guy who has to deal the damage as an ADC, requiring high-DPS mechanics, kiting, dodging etc.
And all of these are topped off by correct itemization, masteries and runes, mostly itemization. It requires a lot of forward thinking on how you are gonna itemize for Gnar. Most people go for Frozen Mallet first, but this can change into a Cleaver or even a defensive item, depending on the game. Even after you get your Maller and Cleaver, which is the most optinal and snowbally buy, you need to think if it's best to buy a GA because teamfights are gonna break out, or are you just gonna stay in lane and buy a more Armour or MR specific item. All these are relevant to all champions, but on Gnar, they are so much more important, because of the fact that he has to fill many roles (ADC, fighter, tank, initiator), and thus having the correct items at the correct time for the correct purpose can be crucial and requires a lot of forward thinking.