Dynamics ISV, eBridge, now has an EDI related blog for Microsoft Dynamics GP. If you’re looking at EDI solutions for GP that’s not a bad place to start. (For the acronym challenged, ISV is independent software vendor and EDI is Electronic Data Interchange)
Dynamics GP EDI Blog
Archived post from 2008. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.