I decided to kill off the leader of the hidden beks gang for the other gang. but i cant figure out how to get to him since he's locked up in his office.
any advice?

How do you mean "locked up" ? He's standing in a pretty big room with a handful of henchman with him, but no more than you would face if you took on the Black Vulkars.

ever since i aggreed to kill him, the beks have got him locked up in his office, since the vulkars have hired an assasin. he's not in the big room anymore

You can do two things, either tell Zaedra (if i remember that name correctly) you're here to kill Gadon, or open an unauthorised door. From what i understand both situations trigger a Beks attack, and you should be able to get to Gadon after that.

well i told them and i have killed them all, but all the doors are still impossible to open. when i try security with mission it says impossible

there shoulda been a passkey or whatnot that you got from the evil gang.

Gadon's quarters are accessible by an elevator, is that the door you can't get open ?