in the exercise, it says " a) In the turtlebot_navigation_gazebo package, search for the launch file named gmapping_demo.launch . You will see that what this file actually does is to call another launch file named gmapping.launch.xml , which is in the turtlebot_navigation package."
I find out that all the files are in the turtlebot_navigation package instead of the “~~-gazebo” package.
This is really confusing at the first place when I try to find the launch and xml and yaml files.
I hope someone could fix it which could help other people who learn this unit