I'm about ready to finish this quest off, however when I go to the tunnels the gate is locked despite me telling Gustavo I'll kill them. Am I now forced in to letting the ghouls in to the tower?

There's a little ghoul sidekick, can't remember his name. He opens that gate for you. Or, kill him, take the key, open it up yourself. Masterston? Anyway, as you enter the station underground, he's up high, above you on the raised walkway. He's friendly, so you can walk up and talk, or walk up and shoot. Go up the escalator u see in front of you.

sure you're going in the right tunnels, there's two tunnel entrances, once is closer to lucky store with the lucky shades, that one is locked until you finish the quest, the other one near the trains where the ghouls are, thats the spot you go into the metro tunnels, and the gate inside where michael masters is should be easy to open, i always kill roy phillips and his friends, i never had that gate inside the tunnel areal locked on me. so not sue what gate you are talking about.