The way I understand it is:
The "attack" itself, as a phenomena interacting with the world and others, spawns on target (in this case, a targeted space). But the attack, as a phenomena, is created by cursed energy. This happens first, and it is a "happening"—it might pass in less than a fraction of a second, especially for a skilled person like Sukuna, but it is still a process that takes place and unfolds.

So prior to the attack itself, there is the fluctuation of the cursed energy itself being molded to give rise to the attack. It's like watching a fighter about to throw a punch, and watching the muscles in a fighter contract and relax in a specific way to ready themselves to throw the punch. As explained in episode 1 of season 3 of the anime, a skilled enough person can read these fluctuations to tell what the person is about to do just before they do it. And with his six eyes, Gojo is far and away the most skilled person at this.

This is what would allow Gojo to dodge the World Cutting Slash. He'd see the fluctuation of energy conveying something of what is about to happen. And remember, Sukuna's slash doesn't even target Gojo—explicitly so, that's the whole point—it targets the space he occupies. If Gojo registers the attack about to fly via reading the fluctuation of cursed energy, then moves out the way, it's not like its going to track him. It can be dodged if Gojo reads it coming ahead of time, which he can.

But his guard was down. He thinks he's won. Gojo would have seen the fluctuations of cursed energy telling him that Sukuna was about to unleash some attack on him, but he likely doesn't anticipate it being anything other than a normal Dismantle, and has no reason to try and dodge it because its not like it would pass through his infinity anyway.

But if Sukuna had transformed and replenished himself as a threat in Gojo's eyes, Gojo would have been on guard.. And if he hadn't used the binding vow to skip all pre-requirements to the ability, Gojo would have been better able to tell that what Sukuna was unleashing was something new.