This is a reposting of an article Chris Roehrich originally wrote on my Developing for Dynamics GP blog.
The Connector for Microsoft Dynamics provides a powerful integration platform that can be customized and extended with the CRM Connector SDK and Web Services for Microsoft Dynamics GP SDK tool sets. The CRM Connector for Microsoft Dynamics GP page in PartnerSource has various downloads to get you started with the CRM Connector including the SDK download. You can find the GP Web Services SDK download here. Additionally, the Dynamics GP Web Services SDK page on MSDN contains the same reference information found in the download page and is available to those without accessing PartnerSource or CustomerSource and can be helpful for quick access.