Drop this in a custom css file
ul.gf-menu {
margin: 0 auto;
display: table;
}

This is how to create a custom CSS compatible with Gantry 4:
Simply create a file called "<TEMPLATENAME>-custom.css" and put this file in the CSS folder of the template (where <TEMPLATENAME> is the name of the template as seen in template manager e.g. rt_afterburner2). GANTRY4 will automatically load this CSS file. If you wish, you can also have browser specific files by appending, for example, "-ie9" making the filename "<TEMPLATENAME>-custom-ie9.css"
More here:
gantry-framework.org/documentation/jooml...custom_stylesheet.md