You are under the gun when you are working under tremendous pressure. Example: "Everyone at the office has been under the gun since this new manager arrived."

You are forced to work extra hard when you are under the gun. Example: "We are under the gun to get this project finished at work."

Working under the gun is working hard under pressure to meet deadlines. Example: "I feel I work harder when I am under pressure." Reply: "Me too. I get much more done when I am under the gun."