Engineers are key on Gold Rush. I placed my sentrygun at the 2nd floor of the 2nd building BEFORE the blue team managed to get the car to the 1st capture point.

The blue team won't have a clue that there will be a Sentry Gun up there cuz they think an Engineer can't get up there.
This will give you a lot of kills (atleast 4 kills) before someone finds out and asks for a medic/demoman/uber etc.

I got 18 kills one time using this trick in just the opening fase of the game. And they just couldn't push through our defense.