If you’re the type of person who doesn’t want to talk with the opposite gender the don’t go up to people and initiate a conversation but if you are walking and someone wishes you a good shabbos you shouldn’t ignore them and act like they are invisible. It might really hurt the other person (Especially if they are more modern and don’t fully understand why you won’t reply)
I feel that when wishing someone a good shabbos is like saying “I hope you have a good shabbos full of rest and inspiration and can recharge for the week ahead” (in just fewer words) why would you ignore a bracha like that(even if from the opposite gender)?