Use this code...
#g-container-main {
margin-bottom: 60px;
}
#g-footer {
position: absolute;
bottom: 0;
left: 0;
right: 0;
}
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.