Chris Roehrich looks at Creating a Custom Serialization Assembly for eConnect for Microsoft Dynamics GP 2010 over at Developing for Dynamics GP.
Creating a Custom Serialization Assembly for eConnect for Microsoft Dynamics GP 2010
Development & IntegrationGP 2010
Archived post from 2010. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.