I found that I had to add the custom font files; MyFont.ttf.svg, MyFont.ttf.eot, MyFont.ttf.woff to the assets directory.
Then add the following to the source file
It will not work in firefox without the "src:local" line.
The problem is that if you input this into GWD source, it removes the first half and only keels the firefox fix.
Is there some other means to add custom fonts that is more straight forward and does not require messing around with code?