Car Throttle Stuck problems of the 2002 GMC Sierra

I started the vehicle after vehicle was sitting for about 4 hours. While backing out of my driveway, the vehicle started accelerating, even though I had not yet pressed the accelerator. Even with my foot pressing the brake pedal as hard as I could, the vehicle stopped briefly, but then the rear tires briefly spun and the vehicle started moving in reverse again, dragging the locked front tires. After 25 feet, vehicle hit parked vehicle on other side of street. Vehicle stopped, but rear wheels continued to spin until I put the vehicle in park . It seemed like throttle was stuck open, even though I had not once hit the accelerator.