So I just got my Transmission replaced in my car, and it starts but when i press the gas it shuts off, the mechanics can't find anything wrong with it, does anyone know what that might be?

have you been getting a vw/audi specialty shop to do the work? or just the average joe blow mechanic? also does the car get past 3000rpms ? if not its most likely a limp mode in the cars engine. Have your car scaned by a professional vw/audi specialist shop and see if they come up with any additional information since most joe blow shops do not have the proper software to properly diagnose problems in your audi.

You may have a mal- adjusted or faulty brake fault sensor. The car has to be started with the brake pedal depressed and this is a saftey stwich. Check it out.