I've seen a lot of activity recently on the Dynamics GP Forum, indicating confusion over what version of SharePoint works with the new GP Workflow 1.0 and 2.0. Quick answer - none of them. GP 2013 R2 and GP 2015 both have a native workflow engine built in, eliminating the need for SharePoint altogether.
Additionally, Reviewers and Approvers need not be GP users to Review and Approve workflow steps if Email Actions is enabled. Approval can be initiated from within email notifications generated by the workflow.