Grand Theft Auto: San Andreas/Up, Up and Away!

Don't kill any one inside the premise. The gate will not open again. You screwed your mission.

There is However, a more creative way of dealing with this mission if you didn't get in the gate.

Go to the airport and Grab a plane. Fly up to a high elevation, and fly to the compound, when you get close, jump out and use your parachute to land on the roof where the helicopter is (rockets will shoot at you, but they will miss because they can't home in on a parachute). from there, go to the stationary gun and take down the hunters that come, afterward, take the helicopter and continue with the mission like you were supposed to do.