jBPM runtime engine could be considered just as a set of jar files. JBoss Developer Studio/JBoss Tools can either
create new jBPM runtime and provide you with such jars, which can be then copied to your GWT project: http://docs.jboss.org/jbpm/v6.0.1/userguide/jBPMEclipseJBPM.html#d0e14020
or, add them as maven dependencies: http://docs.jboss.org/jbpm/v6.0.1/userguide/jBPMEclipseJBPM.html#d0e14069