Gazebo world to probabilistic map based on occupancy grid

Hi everybody, I am new to ROS and I have to do this work. I created a Gazebo world and I created a node with Simulink to move my Turtlebot in the map by writing the position to reach in the topic. Now I want to get the probabilistic roadmap of the world I created in Gazebo so the Turtlebot can reach a point without hitting the obstacles. I would like to do it without scanning the whole world first. Thanks in advance for your help.