The best picks not in lane to counter Gwen include Sivir, Caitlyn, and Kog'Maw. These champions are all great at countering Gwen with high win rates over 60%. Even if these Gwen counter picks are not taken opposite your position, it's not a good idea to take her into any of these matchups. They will usually win against Gwen.
Yet, if the enemy team brings either Ziggs or Fiddlesticks, Gwen mains should strongly consider choosing her, as she easily crushes these champs with win rates over 51% against each ofthem.