The Google API doc page for the Where
class lists the DLL that you need to add as a reference, and the namespace within that assembly:
Namespace: Google.GData.Extensions
Assembly: Google.GData.Extensions (in Google.GData.Extensions.dll)
This works correctly for me in VS2012, using the latest API download (2.2.0) from this page, dated June 2013. MSI hardlink here.
It's not an obvious location if you look at the list of DLLs installed as part of the API setup (the obvious guess would have been Google.GData.Calendar.dll).