BlogScrum has a nice article on HOW TO: Use the Dynamics GP 2010 Web Service
HOW TO: Use the Dynamics GP 2010 Web Service | BlogScrum
Tips & Hidden FeaturesMicrosoft & the GP EcosystemGP 2010
Archived post from 2012. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.