Your Win Condition
-Get priority Level 1 and hit level 2 first -Build a lead pre-6 before Garen spikes with Berserkers -Constantly poking Garen when he last hits with Q > E
General Strategy
You can buffer your Q before Garen's Silence lands. Garen wins mid-game but you outscale him in the side lane and in teamfghts. Matchup is alot harder if Garen is full damage and has Phase Rush
Trading Patterns
Constantly harass Garen with stacked Q's whenever he farms especially pre 6 right from level 1. You are much stronger than him in short trades because you can easily kite him with range/mobility advantage. Keep chipping away at his health before you go for the all-in
Wave Management
Level 1, usually Push in 3/4 waves and go for a cheater recall, then try to keep the wave on your side, setting up a freeze after you've built a lead in this lane
Their Win Condition
-Garen wants to play safe until he buys Berserker Greaves and hits level 6 -Garen is looking for all-in fights when Gwen is below 70 percent hp with Q > E > R> Ignite -Garen is waiting for Gwen to blunder