Gnar can benifit of surprise in early levels to people who dont know that he shapeshifts and this can help catching out enenmys just dont be to greedy in the laning faze because you need the the levle advantige

whene you throw your boomerang if you catch it the cooldown is reduced,so make sure you catch it.
With your E you can jump walls in the jungle.
A great way to engage on a 1v1 is by using your e on an adversairy,land behind them an d use your Q (boomerang)
this works well during laning phase.
when useing your ultimate slam the enenmy champions into a wall.

for team fights try to charge your transformation by hitting minions so you can use your E to engage.
your ult can be useful in getting rid of the others teams tanks ar you can use it to get away after baron or elder drake.