Gai tops because Kakashi wants it this way. He's a gentle soul, eager to please and quick to flinch when criticism is given, and he wants more than anything for Kakashi to be happy. So no matter how Kakashi pleads and begs and sometimes cries, Gai knows Kakashi doesn't really want to be untied, because Kakashi told him so before they began. Sometimes it's hard to roleplay, though. Really hard to pull down Kakashi's mask when Kakashi tells him not to, harder still to lean in for that first kiss.
But when he does, and Kakashi eagerly kisses back, Gai feels better. And he feels better still when the pretend 'no' turns into a real 'yes' as Gai strokes Kakashi's erection in his hand. Feels jubilant that Kakashi is turned on and leaking and so wet. Wet, hot, and hard, pulsing in his hand, hips jerking.
And when Kakashi comes, he feels that rush, a rush at seeing the bliss on Kakashi's unmasked face, the limp contentment.
Finally, when he's carrying Kakashi into the bathroom for a shower, Kakashi is normal again, telling Gai how good it was and kissing him and snuggling up against him. If it weren't for the reassurances, Gai didn't know if he would be able to do it again. But the reassurances come, and they're so sweet, and after the shower they do it like Gai wants to do it and it's all okay. He loves Kakashi, so much.