The page demonstrates how to use the WOFF version of Gentium Plus for web pages, using local copies of the .woff files in this archive. For more information on this technique - and how to use the normal TrueType version as a web font - see Using SIL Fonts on Web Pages.
The body of this document is styled using an external stylesheet, with Gentium Plus set as the default font, and loaded directly as a WOFF file using the @font-face rule. If you're seeing a serif font here then it is working. Otherwise the text will be in a sans-serif or your browser default. Here is some italic and back to regular. This technique should work only in Firefox 3.6+, Chrome 6, the IE9 beta, and in development versions of Safari.
To see specifically how these techniques are specified in the CSS and HTML view the source to this page and the accompanying CSS file.