ros gmapping change scan topic

edit

What you did in your example essentially is the same as typing the following on the terminal (you forgot the "scan:=" part):

rosrun gmapping slam_gmapping new_topic_name

what you wanted to do is:

<node name="slam_gmapping" pkg="gmapping" type="slam_gmapping" args="scan:=new_topic_name" respawn="false" output="screen"/>

Note you can also use the remapping syntax provided for roslaunch (this is preferred as it is more clear). See also the remap wiki page:

<node name="slam_gmapping" pkg="gmapping" type="slam_gmapping" respawn="false" output="screen">
  <remap from="scan" to="new_topic_name />
</node>