There is a sample included at 10 SP2 that allows you use any SMTP server of your choice like GMAIL. You can find it at - <Install Location>\WMX\Developer Kit\Samples\CustomNotifier
Follow the instructions in the ReadMe.htm file to compile the code.
You will also need to open up the solution to change the connection parameters that are available there. This sample should at least get you going, but feel free to modify this to suit your needs.
This feature will be available Out of the Box at the next major release of ArcGIS Workflow Manager.