How difficult ? And how about scenario 2 ?
Mid-high difficulty at the very most. Gai is fast, but he can't do anything to them as long as Itachi or Sasuke can hold Susanoo up, neither can Gaara as long as they are careful and watch his movements so that they don't get caught by his sand. Pulling out of Susanoo won't happen as the sand has to hold Susanoo in place, and Susanoo would simply slap the sand away before it could do that.
So Amaterasu+Katon lets Sasuke use Kirin while Itachi defends them both and then Gai and Gaara get lit up. Either that or Itachi and Sasuke bust through Gaara's Sand defenses using Susanoo (Madara's Magatama alone in V2 ripped through Gaara's Sand Shield and still had enough power to crack Onoki's shield, and the attacks Sasuke and Itachi can dish out with V4 Susanoo are leagues above that).
Scenario 2 is no different. In fact I forgot Itachi was healthy for scenario 1 when I made that post.
