Set Gmail as default mail handler for Mail PDF print option for Snow Leopard
The only way that I know how to accomplish this is to use Mailplane which is a commercial Mac app for using Gmail's web interface.

It has its own "Mail PDF with Mailplane" service which comes as part of the app:



There's no way that I know of to automate the process of attaching a PDF (or any other file) to Gmail through a web browser. You're going to need a Mac app for that. That either means setting up an email client such as Mail.app (which means duplicating contacts, etc) or Mailplane (which will let you use your existing contacts).