how to config SSRS 2005 email setting with google apps
This msdn walkthrough should have everything you need to accomplish what you want to do on the SSRS 2005 side.  Pay attention to the section on setting up for remote smtp.  Especially the part about setting SendUsing to 2.
This google guide provides the steps to configure things on their end.


The setup for SSRS 2008 isn't much different.