Gnar is pretty strong ATM, but you have to remember that he has a few major problems:
A) His base[Non affected by passive] are BY FAR the lowest in the game.
B) He can't really control when he transforms.
C) His mini Gnar abilities hit with the approximate force of a gently swung pillow, especially early game. Although his W, later in the game, becomes more equivalent to the same pillow, but thrown out of some kind of siege engine. Low cooldown abilities means that Rito usually gives them low impact or restrict them some other way[Liss and Urgot Q's have a high mana cost, Karthus's Q is easy to dodge and Sone's abilities all had comparatively minor effects when their cooldowns were lower].
D) Itemizing is sort of hard. Sure, building AS to maximize your % health damage in mini form is cool, but it makes you turn into MEGA GNAR much quicker, which makes that AS item mostly useless. Current meta for building him appears to be TF + BOTRK + a couple tankyesque items like Frozen Mallet, which is as close to a middle ground
Still, Gnar is really strong right now. Not denying that. Mostly because he can go boing-boing-kite AND boing-crush-smash.
Answering other questions:
@W key: The W key is actually just the standard for on-hit passives in the ability slot. Gnar's Hyper, Vayne's Silver Bolts, Teemo's Toxic Shot and Vi's Denting Strikes[<-I THINK?]. Same way that Q is almost always your bread and butter spamming ability and on-target shields are almost always E.
@Immobile awesome casters:
Its almost balanced, really. They get to be more effective, objectively, because they have less mobility. Positioned right, they can be ridiculous. The problem is getting into that position and not getting your face melted off by some upstart young assassin champion.
@Leagues:
I've never been above Bronze I. I've also never played more then ~20 games[Including the starting 10 for promos] of ranked in any given season.