This is how to achieve all the endings in SCP Containment Breach

Gate A Ending 1
You will achieve this ending by failing to recontain SCP106 (The Old Man) or either refusing to. You need SCP106 for a distraction.

Gate A Ending 2
You will achieve this ending if you successfully contain SCP106. I guess we don't need a distraction anymore!

Gate B Ending 1
You will achieve this ending by listening to 079, once you reenable the door controls, SCP079 (The Old A.I) will announce "The Doors to Gate B are now open", you can either go to Gate A or Gate B.

Gate B Ending 2
You will achieve this ending by manually disabling the Warheads. Huh, maybe you should have kept them on, something big is there.