For example, you dont have much time to fully upgrade an event girl and the current event ends in a few hours. With no gems in your stock you decide to buy a few to renew your stamina. This is not pay to win? Paying customers always have the edge over free player.

We will assume said player spends his gems on other things for the sake of this argument, because we cannot speak for every player on how gems should be used and how a person manages his playing schedule.