What you are talking about is a concept commonly known as:
However, not all companies can really be their own customers. For example, consider Microsoft. SQL Server in previous versions were not as powerful as now for enterprise purposes. Thus Microsoft couldn't use it and it had to use something like Oracle, or DB2 for enterprise level developments.
I mean, the first point is that not everybody should eat his/her own dog food.
For example, sites built with ASP.NET and running on IIS have an HTTP Response header field known as
X-Powered-By: ASP.NET, or something like that. But you can easily delete these headers from IIS. Also websites created by ASP.NET usually have paths ending with
.aspx files. However, this also can be easily changed to extension-less URLs with the concept of URL Rewriting.
So the second point I'm emphasizing, is that, you can't ever for sure say that Google is not using GWT. Maybe it does.