If you want to break that gate you have to go to the thing where you can get your vehicle. Get one that has an explosive blast to it then you can blast the gate down and it should unlock the Separatists Assault Menu.
The other assault menu is still locked and I don't understand how to unlock it. I am also missing two vehicles from either side, as well as Grand Moff Tarkin.
I have all red bricks, 93 of 130 gold bricks, true jedi for every mission, all minikits, and all bounty hunter missions are complete. I am not sure what I missed, or where.
Thanks for any help you can toss my way.