only reason gnar gets picked in the lcs and not other tanky tops is that he does ABSURD amounts of damage while still able to build full tank beyond a brutalilizer.
Most tanky DPS champions do absurd damage while building full tank.
Nasus scales his damage just through CSing, and he gets a free 4% max HP/sec shredding aura, along with 40 armor/mr penetration, and a potential 300 AD steroid.
Nasus is a much better definitition of free damage than Gnar.
If you compare Nasus to champions like Shyvana, Mundo, Udyr, Warwick, Darius, etc. - he's not unique in that capacity either, so no - Gnar doing damage without building damage is not something Gnar stands out with.
I think gnar is a spot on example of champs that get picked in the lcs just because they do everything better than what others in his role can do
I really don't see Gnar as doing everything better than the next guy.
I'd go more with Gnar is a jack of all trades, master of none as being the reason to pick him.
It's the same general strength Lee Sin, Nidalee, Lulu, Elise, Jayce, Kayle, and other frequent tournament picks have. They don't excel at any one thing in particular, but rather they have an entire garage full of tools and capabilities to work with, leaving them without much in the way of an exploitable weakness and generally equipping them for any situation without much foresight or settup.
To Gnar's credit, at least both of his forms have clear tactical weaknesses you can exploit while he's stuck in one form, but with proper resource management he basically has no weaknesses you can pick apart with strategies, plans involving longer set-up times.
Champions that clearly rock a category (like Yorick in the lanning phase) tend to be stuck with a single tool for every situation.
Want to weld metal together? You're stuck with a shovel, Gnar's got a soldering kit. Want to hammer in nails? Yorick's still stuck with using a shovel, Gnar's got a hammer. Want to paint the side of a house? Yoirck's still stuck with a shovel while Gnar has a paintbrush. Want to dig a hole? Yorick's well equipped to do that with a shovel, but Gnar at least has a trowel.
The biggest area this impacts is picks/bans, specifically with counter-picks.
A specialized champion is just really easy to counter-pick.
A generalist is not.