If you're asking about the Web application aspect of a Google service, most likely you're going to want Web Applications. See, for instance, questions tagged [gmail]questions tagged [gmail].
There are some exceptions, however.
- Google Analytics is better on Webmasters ([google-analytics])
- Search Engine Optimization also belongs on Webmasters ([seo])
- Google Drive file sync goes on Super User ([google-drive])
- Google also has a smattering of native (non-Web) apps, like Picasa. Questions about them would also go on Super User (e.g., [picasa])
- Smartphone apps go on their respective sites: Android Enthusiasts, Ask Different (Apple), Windows Phone