Can I catch it or know which Lugia has Multiscale in GTS?
Lugia is exclusive to Alpha Sapphire and can't be caught with the multiscale ability no, it has pressure 100% of the time.
Multiscale Lugia was only obtainable through Dream Radar.
Multiscale lugia is now going to be available as an event for preordering the movie ticket for the Hoopa movie. It has the Hidden abilities for 6 legendaries and you get a code for one when you preorder.
if you have soulsilver, dream radar and white 2 or black 2. you can catch it in dream radar with soulsilver in the console and then transfer it to white 2 or black 2 and then put it in Pokemon bank and then in oras.