Custom font on landing pages only displaying on Mac?

We are trying to launch a new landing page template, and have specified a non-standard font to use on the pages (Gotham SSm). However, the font is only showing up in browsers (Chrome, Firefox, Safari) on Mac. Those same browsers on PC (and also IE) show Helvetica/Arial.

Here is the code we have specifying to add the font:

<link href="https://www.instant.ly/css/ce/fonts/149381/AD116F7D4F9AF23F3.css" rel="stylesheet" type="text/css">

<script language="JavaScript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>

<style type="text/css">

@font-face {
   font-family: "Gotham SSm";
}

#bodyId {
background: #fff;
font-family: "Gotham SSm", Helvetica, Arial, sans-serif;
font-size: 14px;
text-align: center;
margin: 0px;
padding: 0px;
}

And here is a WIP landing page test page:

pages.instant.ly/LP---Instantly-Templates_LP---Faux-Site---V2.html

What could be the cause for this issue, and how can we solve it?