firebug or chrome's dev tools "Element Inspector" will help you determine the elements involved.
add this to your custom css file:body .gf-menu.l1 > li.active, body .gf-menu.l1 > li.parent:hover, body .gf-menu .dropdown {border-radius: 0;}
If you are using Gantry 4 (or later) you can create a file using a simple text editor for custom css and name it using this format: templatefolder-custom.css (e.g., rt_kirigami-custom.css or gantry-custom.css).
the above css will deal with the border radius'
always make sure your css is m,ore specific than the css already in effect.
see this:
coding.smashingmagazine.com/2007/07/27/c...ngs-you-should-know/