Here are some comments by Gypsylord about tank Gnar:
I do wish he was a little less hardcountery to certain low mobility melee champs. I didn't really account for the fact that he likes building straight tank in a lot of games which tends to negate a lot of mini Gnar's weaknesses (he's supposed to be easy to kill).
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I think it's good and healthy that Gnar can buy a hexdrinker to not get wrecked by a rumble just as it's healthy for an ADC to be able to pick up a banshee's veil. The problem isn't when he buys one tank item, it's when he buys 5.
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Gnar was supposed to build "one or two" offense items. Right now he builds .5, if that, and then goes straight tank.
If that was true then, why buff base AD and encourage full tank builds even more when it was clearly unintended and makes Gnar overall too hard to kill for his level of mobility and cc?
On another note, Gnar hasn't seen any changes for about a year. He was super popular and did extremely well throughout season 5 because he was super tanky for how much damage he did. Now look at the champs he's losing hard to:

It's like a who's who list of power creep and balance nightmares. With the exception of Teemo, Vlad, and Kennen, these champs encapsulate some of the core complaints about balance this season:
-Tanks do way too much damage without building any damage
-Fighters get too tanky from building juggernaut damage items
-AD is so optimized that AD champs spike hard and fast with few items
-AP items are mostly overpriced and poorly optimized for everyone other than AP tanks
Of the 6 AP champs on the list, 3 don't even bother building AP, 2 others don't use mana, and the last doesn't need mana for the majority of his damage.
The point here is that buffing Gnar's base AD tells me that you either don't know or don't care about how miserable it is to play game after game full of tanky fighters, tanky juggernauts, dps tanks, and now the recent addition of tanky assassins (yi, akali, eve). The level of damage, healing, and tankiness is cranked up to 11 and, frankly, it's godawful.
So why not bring down the overall level of immortality instead of simply buffing Gnar to do more damage while still building....tank. And while you're at it, take a long look at why this is happening in the first place. AD is too strong too early, and Juggernaut items are too tanky with too little sacrifice to damage.