A gunner is an employee who constantly tries to outshine their peers, often by taking on extra tasks, volunteering for overtime, or dominating meetings. While management may appreciate their drive, coworkers usually view them with suspicion. The term implies that the person is willing to sacrifice team cohesion and throw colleagues under the bus to advance their own career.
Heard on the job
Don't tell the gunner about the delay on the supply order, or he will run straight to the foreman with it.