Use this code...
.g-offcanvas-toggle .fa-bars:after {
content: "Menu";
font-family: "Roboto";
font-weight: 400;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.