He is a lane bully and can make the life of melee top laners a very hard one. With his ranged poke and his Q, he can keep chipping on his opponent while they try to farm, and force them out of the lane.
His E allows him to disengage and ruin all-in attempts on him from the enemy team. This makes it very hard for the enemy Jungler to gank him unless the top laner is able to land their CC on him.
His Ultimate R can change the tide of an entire fight. When combined with his E, he can quite easily CC the entire enemy team when they are clumped up together.
Once Gnar’s E is down, he becomes susceptible to ganks, so if his opponent manages to bait it out, he will have a really hard time escaping out of that situation.
Gnar’s Ultimate R won’t be very effective if the enemy is playing in open spaces and is spaced out evenly. This will force him to go for a higher priority target which may involve flanking the enemy team, which can cause him to get caught out and killed.
The enemy can completely counter him by choosing to disengage when he gets his mega form. Once his mega form is down, the enemy won’t be under the looming threat of getting ulted into a wall by him, and hence will be able to fight more freely.
0 - 15 min
If possible, use your range advantage to harass enemy champions from afar when they walk up to last hit. This will allow you to gain a health lead over the enemy.
Keep an eye on your resource bar at all times. Look for aggressive plays when you’re about to change form or when your Ultimate R is about to come up.
When you’re level 6, you actually have a surprising amount of damage. You could use this to your advantage to get a kill or set up your Jungler.
15 - 25 min
Delay team fights until your Rage Bar is fully stacked. Do not team fight until you’re ready to change. Once you’ve changed forms, make sure you engage and fight.
Group as often as you can. Gnar is a really good team fighter in the mid-game as long as he can get his Ultimate R off.
When no team fights are occurring, split push. Your champion is good at split pushing, so you may as well go to a side lane and start to push while your team attempts to siege elsewhere.
Group with your team in the late game. If you split up and leave their side, it’s going to be difficult for them to win the team fight and the enemy may try to force a fight when you show somewhere else on the map.
Play around your Rage bar. Be prepared to force a team fight when applicable if you’re about to transform.
If you’re not about to change forms, delay team fights for as long as possible until your Rage bar is stacked and you can change forms.
Gnar's level 6 is a massive power spike for him as he can completely dismantle an enemy team with a well-timed Ultimate R. He will also be instrumental when it comes to flanking the enemy team during significant fights.
His first item component will give him a decent damage boost. He is already quite obnoxious due to his Q and W. Still, the component will now let him make the enemy recall abruptly quite often.
Gnar is relatively weaker early on because he doesn't have his Ultimate R available. As a result, he can't dish out a lot of damage. To compensate for this, he will want to use the lane brushes and his range advantage to keep poking the enemy champion while using his E as a measure to maintain the spacing between his enemy and him.
This is the point of the game where grouped fights become common. This means that Gnar can now decimate the enemy team with proper usage of his Ultimate R.
At level eleven, he will have two points in his Ultimate R. This is really good as it increases the overall damage dealt by the ability and reduces the general cooldown.
His Q is his primary poke tool in the lane and in the game. It will be maxed out at level nine, ensuring that he can harass enemies relentlessly and force them to recall abruptly.
Gnar can deal a lot of damage or be a peeler for his team by tanking a lot of damage. This versatility depends on his form at the time of the fight as well as his build choice.
At level thirteen, he will have two of his abilities wholly maxed out. That will allow him more frequent usage of his jump and poke abilities which can be highly beneficial during extended fights.
Gnar must play around his Ultimate R to ensure that he is vital during the late game. Bonus points if he can use the fog of war to his favor.