Well hers's a short breif on wat I think.
Balamb Garden could fly cause it was remodeld to fly. And that Galbadia Gerden flew cause of the sorceress. Cause Cid didn't know hardly much at all about it, so who informed Galbadia about it? I just think it was the sorceress magic. Cause if all 3 could move, why didnt Trabadia I think thats how its spelt, move when there missles were coming torwards them? Yes I do know they wasnt warned, but with that technology u know they got to have some sort of scanners that would have seen the missles cooming, and yes again I know the missle are supposivle stealth missle (I think) but you can change the missle that are shot off. So they would have picked them up and moved, but didnt cause they couldnt. So once again, Balamb only moved cause it was remodeld too, Galbadia moved cause of the sorceress. If you pay attention to waht Cid says, he didnt say the gardens were used as shelters and were then remodeld into able to transport, he used the words "this garden"
And the Shumi didnt use technology to help remodel, he simply gave money to supprt the Garden.