The page demonstrates how to use the WOFF2 version of Gentium for web pages, using local copies of the .woff2 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 set as the default font, and loaded directly as a WOFF2 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. Styling can be done with HTML tags: Here is some italic, some bold text, some bold italic and back to regular. It can also be done by referring to specific CSS weight axis values:
This is 400 (Regular).
This is 500 (Medium).
This is 600 (SemiBold).
This is 700 (Bold).
This is 800 (ExtraBold).
To see specifically how these techniques are specified in the CSS and HTML view the source to this page and the accompanying CSS file.