It's going to be really hard to help you if you don't get your site online. This issue is all about context in which you are doing this - what happens depends on the context and what other classes you have used. But, i've guessed and I think this will help you...
ul li.fa {
font-family: inherit;
display: block;
}
ul li.fa:before {
font-family: fontawesome;
}
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.