You want to change the width of your Gantry template?
Here are some template specific solutions.
They include some individual css tweaks for each template.
Omnicron
www.rockettheme.com/forum/index.php?f=30...09340&rb_v=viewtopic
Panacea
www.rockettheme.com/forum/index.php?f=30...04348&rb_v=viewtopic
Juxta
www.rockettheme.com/forum/index.php?f=30...12769&rb_v=viewtopic
Quasar
www.rockettheme.com/forum/index.php?f=30..._v=viewtopic#p524380
Crystalline
www.rockettheme.com/forum/index.php?f=30...05519&rb_v=viewtopic
Here is a general solution.
Probably you need to tweak the css a bit when you use this.
In most templates background graphics need to be adjusted.
(won't really work in Hybrid and Paradox because both templates use a different mainbody-layout)
Template independent solution for 12-column-layout of gantry
(But not for Hybrid, and Paradox because of mainbodylayouts used here)
www.rockettheme.com/forum/index.php?f=30...13589&rb_v=viewtopic
Template independent solution for 16-column-layout of gantry
(Thanks to David Goode for adopting my code for this)
www.rockettheme.com/forum/index.php?f=30..._v=viewtopic#p571846