They are just absolutely brilliant together. And I think it's a combination of bouncing off each other's spontaneous reactions well, but also having a plan and complete synergy in how they understand their characters and the arc of the scene. It all works together to secure the Phantom's transformation and redemption while also leaving the audience feeling like Christine made the right decision, which is such a hard line to walk in the final lair.
I think Gina's Christine genuinely loves Erik in this one, but there was absolutely no way she was going to stay with him after everything he'd done. The decision is questionable for other Christines, but her particular take on the character just wasn't going to allow for that, especially given his Phantom's more aggressive approach to the final lair.
And unlike other interpretations of this scene, both she and David/the Phantom know that and it plays out in their body language post-kiss. Up to that point, David had literally been grabbing Christine at will and throwing her all over the place, all while desperately denying that his actions were dooming their relationship. And now she comes back to return the ring and he literally can't make himself touch her because he knows he's pissed away every claim to that kind of intimacy with her. There's no verbal or physical bid for her to stay; that unfulfilled caress says "I recognize what I've done and that I have no right to ask you to be with me, so I'm letting you go." And even worse is the fact that David also plays Erik with chronic pain, so we can hardly tell if the face twitch/wince he does is literal or figurative (let's just say it's both).
GIVE THEM A DAMN OSCAR, IDC.