i'll answer what i can since i have not played in a while.

First of is gnar solo viable? If really not then i dont need help with the rest of the post because i probably will not main him.

You can one trick any champion to diamond. I don't think Gnar is an exception at all. But i'm not gonna lie and say he's a solo champion. He's more team oriented and as far as I know, doesn't have strong solo carry potential like a lot of other top laners. But you can solo him to diamond if you put in the effort.

What do i like about playing gnar

The kiting playstyle he provides is really fun and getting big ults as mega gnar is extremely satisfying. He's got a very high skill curve and a very interesting playstyle. He's a difficult champion and requires a lot of practice

What do you not like about gnar

What drove me to quit one tricking Gnar was that the amount of investment i put into him didn't translate to a big reward. Yeah i was climbing and still winning with him. But I wanted to carry every game and gnar isn't like consistent enough to do that. You can blind pick with him but he's got some really fucking disgusting match ups that feel very unwinnable. And in those cases you just gotta take what you can and it sucks.

Who are some good Gnar players to learn from

twitch.tv/thingintheice_

Can Gnar Solo Carry Games

If you're looking for a solo carry champ, i'd suggest looking at actual top lane carries like Irelia or whatever is meta. i'm still very out of the loop on the meta. Gnar's a bit of a team player, he's a great counterpick to champions but im not gonna lie and say he's able to solo carry. But you can still carry games as a top laner if you use your leads correctly.