$ sudo apt-get remove gns3 

Uninstall gns3 including dependent package

If you would like to remove gns3 and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove gns3 

Use Purging gns3

If you use with purge options to gns3 package all the configuration and dependent packages will be removed.

$ sudo apt-get purge gns3 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove gns3 

See Also,