for navigation inside gazebo, You need to model the drive wheels links with joints but if the lidar is mounted directly to the base link you can simply place the sensor on the baseline. Later after that works make a lidar base and scanner and place the sensoron the scanner.
The idea is to do the minimum amount of work to make "rev 0.1" do anything at all them incrementally improve it. If it wereme, I'd leave the lidar off the urdf file and not use the nav stack either. Just drive it with keyboard teleoperation