The repository shows that the file you are (probably) looking for is at a different path: you are missing a gmapping/ string in the include. So, in your a1.launch file you should change the line you have to:
<include file="\$(find turtlebot_navigation)/launch/includes/gmapping/gmapping.launch.xml"/>

Instead of going to the repository online you could also use roscd turtlebot_navigation and find the path to the file you wish to include from your local filesystem.