For one, it should be said that receiving flak for your screenname and receiving flak for your builds are two marginally different things. They both suck, and I'm sorry you have to deal with it, but hopefully you at least mute the players once they start in, and report them post-game.
For receiving flak for your screenname - it's kinda just a thing of Yasuo and Yasuo mains having a bad rep. Yasuo is one of those Champions who, due to his high skill ceiling and moderate skill curve, attracts some unfortunately toxic players to main him.
Of course, not everyone who mains or plays Yasuo is toxic by a longshot, but, for some reason, katana-wielding swordsmen seem to attract some bad folk. Dunno why.
For receiving flak for your build - you're building Attack Speed on Gnar, which, in and of itself, isn't a bad build, it's just really uncommon. Gnar may be a tank/bruiser primarily, but he is also a marksman, and his kit does facilitate both tank and marksman playstyles, albeit, for obvious reasons, the tank style is much, much more popular.
Don't let people discourage you from building what you want, though. If you want to build attack speed and capitalize on your Hyper, go for it. Nobody can force you to play what they want. so if they start chewing you out for building AS Gnar, just mute them, report them post-game, and enjoy your 100% Hyper uptime and damage bursts.