Gwen vs Garen Top Matchup
Gwen Top wins 46.5% of 1,608 Low Elo games against Garen Top, -1.1 points vs expected, 2-patch window 26.19-26.20.
Researched on LoL patch 26.20. Tips are written to hold at any rank; stats come from Low Elo ranked games up to patch 26.20.
How to play as Gwen
Use max-range Q poke and keep E to retreat when Garen runs in with Q; avoid dashing onto him unless the full trade can finish him, since his Q and E can turn an extended exchange.
Stay healthy enough to avoid giving Garen an easy Q-E-R burst window; trade around his approach instead of trying to force an all-in through his silence and damage reduction.
Garen Q is an empowered basic attack, and Gwen W makes her untargetable to enemies that remain outside its mist; Garen can still reach her by entering the mist.
Sources
These notes come from r/GwenMains, checked against the champions' abilities.