Kamui's warp point is quick but the warping action is not. Sand can be thrown at the warp point for a good measure to save the individual. Konan manipulated this factor, as even Obito's object-warping is slow compared to his intangibility. If Obito, whose S/T ability is hailed to be better than Minato's cannot accomplish so, then I can only imagine how it's like for the Uchiha scrap licker. It's a fifty fifty affair, honestly.
I'm gonna break down everything here.
His warp is instantaneous, it warped so fast that the BM Naruto clone had already poofed away into smoke, it was seen as dead from even sharingan standards.
Kakashi's kamui allows him long range warping. Obito's allows him short and the ability to suck and decompose things from the other dimension.
How konan manipulated it was, she knew that he could become intangible and when he wanted to attack he materialized it, and she figured out his time limit. A genius really, but kakashi's doesn't work like that.
Kakashi's doesn't have a time limit. He's been using it multiple times during this war and even with some chakra gained, when it deplinished he was still able to do it.
Gaara's sand is one of the slowest things in the series outside of Sunagakure, the fact of the matter is it is fast once it catches you, it will not be able to catch kakashi.
Several scenarios.
-Gourd is kamui'd.
-Sand is kamui'd.
-Blitzed with Lightning Transmission.
-Sand is dismantled by Raikiri.
-Lightning Transmission is kamui'd directly into gaara, this or raikiri beast running, or he can kamui a raiton clone in for a suicide bomb.
-Raiton Clone can be kamui'd to stick gaara underground.
Low diff gg lol