RE: Motor smoked
I wouldn't think running on gravel would have put any kind of undue strain on it. But, it's possible that a piece of gravel got caught in one of the drive wheels, causing drag, and that might have over-heated the motor.
In any case, it should still be under warranty, so contact the place you bought it, and get a new motor.
I also strongly suggest you get an IR thermometer and use it to monitor your motor temp while running. It will tell you if the truck is over-geared, which is another possibility. A brushless motor should never get over 180 degrees. Keeping it under 160 will prevent damage. Nominal running temps are under 150.