That's the neat part. He didn't
Nuh uh. Whis was moving backwards, he was in front of where Gogeta was supposed to teleport, but moved back so Gogeta was in front and could fight.
If you superpose both images (I used the background for reference btw that's why the lower part is cut) you can see Gogeta is not as close to the screen as Whis is before moving out (Gogeta's body can be seen entirely except his feet, while you can't even see Whis' hair previously to him moving).
Not to mention, as I told you, there is no proof of Instant Transmission teleporting one inside other people instead of its surroundings. No one in Dragon Ball has ever done such, even when they were at risks with the planet at stake. And why would Gogeta even want to kill Whis by teleporting inside him on the first place? Why risking teleporting inside Whis and making him evade (without knowing if he even could) instead of just teleporting near him like every other time instant transmission has been used in the series?
The argument makes no sense. Not only there is no proof of Instant Transmission working like you claim for Whis to evade, but there also is no proof of Whis evading anything. And I haven't even started pointing out how this isn't immeasurable speed.
Also, iirc in Cell Saga we saw that Ki attacks can already travel some distance mid-IT. So it's meaningless anyway