no - if a user has clicked to change layout mode to 1200pxfixed then the body tag will have a class of "layout-mode-1200fixed" on it. therefore you can then but a link on you page
<a class="myrevert">switch back to responsive</a>
and style it with CSS
body.layout-mode-1200fixed .myrevert {display: block;}
body:not(.layout-mode-1200fixed) .myrevert {display: none;}
Obviously you'll need to deal with all the other variations too.
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_fracture). 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"
Regards, Mark.