A gun or a starting gun is an object like a gun that is used to make a noise to signal the start of a race.
发令枪
To gun an engine or a vehicle means to make it start or go faster by pressing on the accelerator pedal.
把…发动起来; 给…加大油门 • He gunned his engine and drove off. 他把引擎发动起来,开车走了。