Aegis of the Legion on Gnar?

You shouldn't pick Gnar in the first place. He's useless.

Better choices than Gnar: or pretty much anybody is better than Gnar.

But aegis will work on some of the above mentioned champions, especially if the enemy team has a lot of AOE AP poke.

It's best to leave that item to support or jungler because the stats are good for the team, but bad compared to banshee's or spirit visage. If you are in a game and your team needs one desperately go head. Otherwise much better choices are out there.

Gnar is not useless at all ignore that shit.

Aegis of the Legion is not bad on Gnar, as all the stats are useful to him. Speaking in terms of the meta, he is a front line bruiser/tank with strong aoe cc and damage. To do this, he needs to be in the middle of the enemy team. All of the stats help him do this whilst also giving his team a nice aura to sustain through an aoe comp.

However, if you are against a champion who deals primarily magic damage, look at your other itemization pathways. Aegis of the Legion gives a mediocre amount of health and mr for 1900g. Spectre's Cowl gives better stats earlier, and it also upgrades to more useful solo lane items. The same goes for Hexdrinker. While not being a good first or second item, taking a look at it when you have three or four items would certainly not be a bad idea.

Situationally yes, situationally no.