Whats better scleral buckle or gas bubble for an retinal detachment? Does the gas bubble move around?

It depends. A retinal surgeon may chose to use either or both in a surgery to repair a retinal detachment. Each has pluses and minuses. A scleral buckle can change the shape of the eye, making it more nearsighted. A gas bubble can cause the eye pressure to increase. Gas bubbles move with the eye and will gradually be absorbed by the body.
Depends. Depends on where the tear is, the size of the tear, and the surgeon. Yes the gas bubble will move around, but eventually will dissolve.