Simple URDF vehicle model

Hi, I have been using Gazebo for a few days and I need to simulate a wheeled vehicle, guided via keyboard, housing a GPS and an IMU. In order to do that, I found and followed this tutorial, explaining how to create and simulate an URDF model of a robot: https://www.generationrobots.com/blog/en/2015/02/robotic-simulation-scenarios-with-gazebo-and-ros/ However I have noticed the following problems: 1) the graphic simulation is not consistent with the keyboard input commands: a "real time response" occurs only after many minutes from the simulation start. 2) I added a GPS to the "mybot.gazebo" file, using the following code: 110.0chassismybot/gps_fixmybot/gps_fixvelocity5.0 5.0 5.00.1 0.1 0.10 0 00.1 0.1 0.1 However, despite not having encountered any error/warning from the GPS plugin, the /mybot/gps_fix and /mybot/gps_fixvelocity do not appear on the rostopic list. Does anyone know how to fix these problems? Thank you in advance