Gopi thinks her Ahamji is just acting as if he forgot. She knows he is a straightforward person. If he is pretending not to know, there is a big reason behind it and the only reason can be he is regretting what happened (he will be acting differently if he was just being shy like after the kiss scene). If that's the case, why should she confront him? To add insult to injury? He may then tell her frankly that it was a mistake and he didn't want to embarrass them both by talking about it. All her fears will be confirmed and she will be even more broken.
Let's say Gopi knows Aham forgot. She doesn't know the reason but she is aware he doesn't remember. Again, should she tell him? Not necessarily. This girl was badly hurt by Ahamji. He is acting nicely to her now, but with him forgetting those moments, all the doubts, insecurities and uncertainties of the first year are bound to be back in her mind with full force. If she confronts him, what if he doesn't believe her? What if he says that's fine it happened, but he doesn't want it to happen again. After all, he hasn't made any conscious effort to be intimate so far. Should she invite that kind of embarrassment, humiliation, and heartache again?
Gopi is not one to put people on the spot. She always takes the blame for other people's misdeeds. In this case too, she is going to be quiet thinking that it was her bad destiny that played a game with her (she said as much to her kanhaji) and if she says something to Aham, she will just be putting herself out for more misery and pain. I know we want her to say something, but her defense mechanism has kicked in here. A confrontation is not consistent with her character and in the current circumstances.