The core part (IoC container, JCR, portal data model, backend services) of both are identical. JBoss Enterprise Portal Platform provides extensions (such as out-of-the-box integration with other system)
In term of license, GateIn is LGPL while JEPP is not LGPL
Red Hat is adopting a similar model as Fedora versus Red Hat Enterprise Linux but for middleware software.
GateIn is a project and JBoss Enterprise Portal Platform is a product.
Hope this link helps: