Wikia

Q: On the turn this creature was put into the battle zone, can it attack your opponent?
- A: Yes, because it has the Speed Attacker ability. However, on any turns after this, you can't attack untapped creatures, only tapped creatures or your opponent as usual.
If you put Victorious Gaial Kaiser into the battle zone, and it links and awakens into Gaial Ore Dragon, the Vainglory in the same turn, Gaial Ore Dragon can attack untapped creatures that turn. (Combined with Gaial Ore Dragon's abilities, Gaial Ore Dragon would be able to attack and destroy all of the opponent's creatures.)

Victorious Gaial Kaiser/Rulings