Gaia Building Blocks are reusable UI components that are used to build the UI for the apps in Gaia. They are also referred to as Firefox OS Building Blocks. Since Gaia is open source, you can use the same stylesheets for your Firefox OS applications and regular web apps. There are different stylesheets for different components in Gaia's GitHub repository under the Mozilla-b2g organization.
Note
You can access the style sheets on GitHub at https://github.com/mozilla-b2g/gaia/tree/master/shared/style.
You will notice that along with the stylesheets, there are directories with the same names as stylesheets, such as buttons, lists, and scrolling. These directories contain assets, such as images and HTML code fragments. You can simply copy these assets to your project in order to use them. You should also keep in mind that Gaia building blocks are not cross-browser compatible, and they don't work in Safari and Internet Explorer (compatibility problems in the Gaia Building Blocks...