If you're asking about the Web application aspect of a Google service, most likely you're going to want [Web Applications](http://webapps.stackexchange.com). See, for instance, [questions tagged [gmail]](http://webapps.stackexchange.com/questions/tagged/gmail).

There are some exceptions, however.

* Google Analytics is better on [Webmasters](http://webmasters.stackexchange.com) using [the [google-analytics] tag](https://webmasters.stackexchange.com/questions/tagged/google-analytics)
* Search Engine Optimization also belongs on Webmasters using [the [seo] tag](https://webmasters.stackexchange.com/questions/tagged/seo)
* Google Drive file sync goes on [Super User](http://superuser.com) using [the [google-drive] tag](https://superuser.com/questions/tagged/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., [questions tagged [picasa]](https://superuser.com/questions/tagged/picasa))
* Smartphone apps go on their respective sites: [Android Enthusiasts](http://android.stackexchange.com), [Ask Different (Apple)](http://apple.stackexchange.com), [Windows Phone](http://windowsphone.stackexchange.com)