I will explaine why..

First of all, kakashi's speed> young sasuke (he said that without that speed he couldn't make chidori) and young sasuke's speed > gaara's defence, so kakashi's speed >gaara's defence. Besides gaara hasn't shown any improvement feats with sand (defence, his atack has improved a lot, but his defence still the same).

Long range atacks are not the right ones for atacking gaara, because his sand can block any ninjutsu, but this is a tricky one, kakashi has shown to have one of the best taijutsu (top 10 IMO).

Dont forget kakashi has knowledge about gaara's jutsus, and gaara has none, and don't also forget kakashi uses a lot of clones for fighting, so it wouldn't be a surprise if gaara can finally get him with a sand atack and it is just a shadow clone. Another high point to kakashi is that he can use high range water jutsus, these kind of jutsu are bad for gaara.

And the last point, kamui, kakashi has shown instant warping feats, so with kamui the battle wouldn't last more than 0.5 seconds, and don't say gaara would block kamui with sand, kamui would warp the sand and everything (don't forget kakashi was about to warp gedo mazo's head), besides the battle would go like this:
-kakashi-( activates his MS)
-Gaara: oh a new eye, i wonder what's its abilitie (he would try to block it, he doesn't how how it works)
Bam, end for gaara