Across 592 ranked games on patch 16.21 in Master and above, Gwen mains win 52.7% of their games in the top lane.
Their most common build appears in 44.9% of those games and wins 56.8% of its 266 games.
It starts 
Doran's Ring and 
two Health Potions, completes 
Dusk and Dawn, 
Shadowflame, 
Rabadon's Deathcap and 
Void Staff in that order, and takes 
Plated Steelcaps.
Two other builds are played often enough in the top lane to be measured on their own.
In the top lane, Gwen mains fare best against 
Jax (won 77.8% of 18 games), 
Olaf (won 73.3% of 15 games) and 
Ambessa (won 72.7% of 11 games).
Their hardest matchups, the champions that counter Gwen, are 
Camille (won 40.0% of 15 games), 
Renekton (won 50.0% of 14 games) and 
Yone (won 50.0% of 14 games).