You just can't do that. gantry.css file is used by Gantry framework which our templates use and in order to change that you would need to change core implementation of gantry framework
when i create a simple menu , the css code is in the gantry css file ( i use firebug to see it)
but the ganty.css in a folder that is not easy to access with dreamweaver so i copy the menu css code /* Lists */ in the template css style1.css
i hope it will the best solution to manage easely my css code to gather the code css that i use to change in a simple css and to leave the css that i don't change in the gantry css file