Anyone know of a good Gnar guide?

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Mostly how his kit is supposed to work with mega gnar. I get how you're supposed to build up rage, but do you only all in with Mega Gnar after you've poked with mini-gnar, and how good of an idea is it to all in with mini gnar using the e and q?

It's best to try and manage your rage the coordinate with jungle ganks when you want to all in. Definitely poke them out with your Mini Gnar Q, and focus on using your E more for positioning yourself when you're in a sticky spot. Make sure you're aware of your surroundings when using Hop as well - if you're getting ganked and you have minions in your escape route, bounce off them for the extra range. You should only use it to go in after a fight when you need the gap closing to finish off a kill. Outside of jungle ganks, only go all in when you can be sure their jungler won't show up, when the enemy laner is low enough, and when you have your ultimate up. I once escaped a failed all in when the enemy jungler showed up because I had my ult up and used it to stun both of the enemies.

Is mini Gnar's kit supposed to be centered around attack speed (for the w proc), while mega gnar is just about sustain?

Mini Gnar is much more about poking the enemy with your Q and using your W and E to either pursue/evade enemies. A little known fact about Gnar's passive is that his passive gives him more bonus stats besides attack range. Mini Gnar has more movement speed and attack speed, while Mega Gnar has more health, armor, magic resistance and attack damage. The difference in stats is significant enough to make all in-ing as Mini Gnar in lane a risky move. So with Mini Gnar, it's best to play like a mage/adc and hang out in the back lines.

Until you go Mega. When that rage bar fills up, you should be ready to hop into the fight and use your new bonuses to wreck the enemies. Mega Gnar isn't about sustain at all - he's about a thing called "windows of power." A "window of power" is an opportunity where a character has a significant increase in power, but only for a short time. Mega Gnar is a big window where you can drop lots of mutli-target crowd control and area of effect damage onto enemies, all while soaking up damage himself. Using the 15 seconds of Mega Gnar wisely can make or break the game.

He's also got another window of power, this one not as big or well known: as soon as he reverts to Mini Gnar, he gains a movement speed burst equal to the same amount his Mini W would give. That can be used after big fights to chase down that last kill or get out safely, if needed.

I've played a lot of bursty mages/assassins in the past, and I'm getting the impression that Gnar's kit isn't supposed to work like that, so it could just be a learning curve, but any tips would help.

Assuming this is your first fighter/tank experience, you've still got a lot to learn about being a tanky front liner who doesn't always deal damage. But Gnar is a good starting point because he can transition between a back line damage dealer and front line tank throughout the game. Although, I'd recommend staying on the front lines because his range is not the longest, plus he needs to land a good Hop/Crunch in order to get into the enemy back lines.

I'll be here to answer any further questions you have :)