the jbpm-gwt-form jar file is included in jbpm-gwt-console-server.war. So to build the installer from source you would need to run the build with following maven command:
mvn clean install -DskipTests -Dfull
once it successfully completes coppy following libs from jbpm-distribution/target
jbpm-VERSION-bin.zip
jbpm-VERSION-gwt-console.zip
jbpm-VERSION-human-task-war.zip
into jbpm-installer/lib
Once you have that you can run ant install.demo.noeclipse directly or put some already downladed artifacts into jbpm-sintaller/lib forlder such as jboss as7, birt package, designer, guvnor etc. For up to date version number please check build.properties of the jbpm-installer.
birt-runtime-3_7_0.zip
designer-2.2.Final-jboss7.war
guvnor-distribution-wars-VERSION.war
jboss-as-7.0.2.Final.zip
jbpm-VERSION-form-builder.war