The replays in the gallery are primarily actual adult scenes, only the Chapter 1 replays aren't full adult scenes.
For Grace specifically, you need to ask her to show her breasts when you win, that's the key choice that unlocks her content.
And yes, there are real adult scenes at Avril's party as well.
Also, two of the replays in Chapter 4 are actually the same scene, but the girl acts differently depending on what you did with her in Chapter 3. I initially hesitated about whether to include both versions as separate replays or just the first one, but ultimately decided players should be able to revisit both variants if they want to see how their choices affected the scene.