unit moves to open battlefield, can i frigid touch it, then gust it

Yes — play Frigid Touch first, let it resolve, then Gust the unit. Both are Reaction spells, so you can sequence them in whatever window opens after the move (the move itself can't be reacted to, but it stages a Non-Combat Showdown on the open battlefield where spells can be played).

The one catch is targeting: Gust requires a unit at a battlefield with 3 Might or less, and that restriction must be met when you play Gust — you can't play it while Frigid Touch is still on the chain and expect it to work (FAQ #4993). So the unit needs to have started at 5 Might or less (5 → 3 after Frigid Touch). Also note Frigid Touch doesn't kill by itself, so the unit is still there to target once its Might drops.