Ok so I have a 11 impala lt new trans but when I mash on gas car slowly moves

Hello, Sorry to hear about the issue, I think there is a good chance it is a simple fix, though. Does it drive normally when it is not at full throttle? As in, driving regularly around town, maintaining a steady speed on the highway, etc. If the vehicle makes normal power when you press the gas pedal except for full throttle, you may have an unadjusted throttle position sensor (TPS), or the TPS may have a dead spot at full throttle.