The top champions outside of lane to counter Garen include Caitlyn, Zeri, and Samira. These picks are all good at countering Garen with high win rates over 54%. Even if these Garen counter picks are not chosen in your lane, it's not smart to play him into these matchups. They will often crush Garen.
Conversely, if the opposite team brings either Rumble or Ivern, Garen mains should seriously think about selecting him, as he easily beats these champions with win rates over 54% against each ofthem.