Gnar is kind of OP at the moment simply because he follows the design philosophy of a lot of the newer champs; instead of specializing hard in one thing, he can do a pile of things reasonably well, and with items can do them nearly as well as a champ that can only do that one thing. He has a lot of mobility, he has good poke, he has circumstantially good all-in potential, he has a certain amount of built in sustain thanks to his transformation (not to mention tankiness), his actual damage is strong enough for him to make a passable ADC and his abilities are all practical. Yasuo and Braum are both really similar, with kits that cover nearly all the bases. Compare them to older champs like Veigar, whose entire kit comes down to "Hold person in place, hit with lots of damage", Garen, "Be a tank with some peel", and old-Soraka, "Heal the carry, provide the carry mana, poke a little bit". The meta really strongly favours mobile, multi-purpose champs also, so a lot of otherwise solid, formerly balanced champs are useless since there is a multi-purpose champ that can do their job and more if necessary.
In other news, I tried Fiddlesticks mid and he seems decent but his all-in from mid is a bit underwhelming and I always get dominated by assassins when I play him, so I'm looking for a new solid mid. Since my current (narrow) mid pool lacks mobility, I figure I could use a mobility champ, and at the moment I'm leaning towards Kassadin. He seems to have a lot of potential if used right, and he isn't insta-ban anymore, plus I get kicks from the idea of having two nearly opposite champs (Kass and Malz) as my mid mains.