SimpleGMF is a metamodel for the abstraction of GMF diagrams. At some point in the future I will write up about this approach.
java.lang.ClassCastException: org.eclipse.gmf.gmfgraph.impl.NodeImpl cannot be cast to org.eclipse.gmf.gmfgraph.Connection
An Edge is erroneously being generated as a Node in the .gmfgraph. Make sure that the GMFConfiguration > edgeTypes reference is set correctly.