Across 560 ranked games on patch 16.21 in Master and above, Gnar mains win 50.2% of their games in the top lane.
Their most common build appears in 43.8% of those games and wins 48.6% of its 245 games.
It starts 
Doran's Blade and 
Health Potion, completes 
Trinity Force, 
Black Cleaver, 
Sterak's Gage, 
Randuin's Omen and 
Wit's End in that order, and takes 
Plated Steelcaps.
One other build is played often enough in the top lane to be measured on its own.
In the top lane, Gnar mains fare best against 
Jax (won 70.0% of 10 games), 
Jayce (won 63.6% of 11 games) and 
Pantheon (won 46.2% of 13 games).
Their hardest matchups, the champions that counter Gnar, are 
Malphite (won 25.0% of 12 games) and 
Fiora (won 33.3% of 18 games).