
Reviewed for patch 7.3d

Favorable matchup
Reviewed for patch 7.3d
Gnar fights in two forms and the danger arrives with Mega Gnar: Wallop stuns in an area and GNAR! stuns against walls. Wukong has the tool to get out of that moment: Warrior Trickster turns him invisible, dashes him away and leaves a clone that keeps attacking.
Warrior Trickster makes you invisible, dashes you in a direction and leaves a clone that attacks, mimicking Golden Staff and Cyclone. When Gnar transforms, you leave Wallop's or GNAR!'s area and hand him the clone instead.
Boomerang Throw damages and slows whoever it hits and comes back to Gnar; if he catches it, it recharges sooner. Outside its path it doesn't slow you, and every throw that misses is a trade Mini Gnar can't open.
Nimbus Strike takes you to the target and gives you attack speed, and Cyclone spins, gives you movement speed and knocks up the enemies it hits; it can be recast shortly after. With Gnar in his small form and without Hop, it's your moment to go in.
Go in when Gnar has just turned back into Mini Gnar or has spent Hop. When he has been trading for a while and could transform at any moment, hold Warrior Trickster and stay away from walls.
If you're playing Gnar, don't transform next to Wukong while he still has Warrior Trickster: wait for him to spend it. Keep Boomerang Throw for when he comes out of invisibility and don't aim Wallop or GNAR! at the first Wukong you see, because it might be the clone.

Favorable matchup
Reviewed for patch 7.3d
Darius wins up close and Mini Gnar is the weak form. But Rage Gene charges when dealing and receiving damage, so every trade also brings Gnar closer to Mega Gnar. The plan: short hits with Apprehend and Crippling Strike, then back off before he transforms.
Apprehend pulls enemies in with the hook of your axe, and its passive makes your physical damage ignore part of their armor. Mini Gnar gets away with Hop: if you wait for him to spend it, the hook brings him back into your range.
Crippling Strike empowers your next attack with bonus damage and a large slow. After the hook, the slow stops Mini Gnar from getting back the distance he needs to poke with Boomerang Throw.
Rage Gene builds when dealing and receiving damage. A long trade gives you Hemorrhage stacks, but it also fills his rage, and at max his next ability turns him into Mega Gnar, tougher and with stuns. Hit, land Decimate and walk away.
The early levels, with Gnar in his small form and Hop spent. Leave the trade before it drags on and come back once Mega Gnar has ended.
As Gnar, don't give Darius trades in your small form: poke with Boomerang Throw from range and keep Hop to get away after the hook, not to go in. When you're close to max rage, take the fight then: Mega Gnar is the form where you do want melee range, and next to a wall GNAR! stuns.

Execution matters
Reviewed for patch 7.3d
Camille can beat Gnar with a well-timed engage: Hookshot once Mega Gnar has ended and her ultimate so he can't escape. If Gnar is near a wall and can use GNAR!, don't go in: the wall you use for your hook is the same one that turns his shove into a stun.
Hookshot pulls you to a wall and, when you leap off it, knocks up enemy champions where you land. Against a Gnar who has just gone back to his small form, it's an engage he can't answer with Wallop or GNAR!.
The Hextech Ultimatum dashes to a champion, anchors them to the area and knocks away other enemies. Against Hop's leap, it's what stops Gnar from getting away once the fight has started.
7.3d raises Camille's attack speed per level from 0.016 to 0.028 so her combos flow better. Precision Protocol empowers her next attack and can be recast: with more attack speed, the combo after Hookshot comes out sooner.
The window is right after Mega Gnar turns back into Mini Gnar. While he's still in Mega form or fighting against a wall, hold Hookshot and last-hit with Precision Protocol.
Playing Gnar, the way to punish Camille is to transform near a wall: GNAR! stuns her against it. In your small form, keep your distance with Boomerang Throw and save Hop to leave The Hextech Ultimatum's area when it ends, not to go in.