Entities defined to be abstract are still created during the scaffold process in GWT. In addition, this entities are declared as abstract in the Java Code, hence, it creates compilation time problem cause the generated code in the scaffold with attempt to instantiate them.
The following script reproduces the problem:
entity --class ~.server.elements.AbstractElement --abstract
web gwt setup
web gwt all --proxyPackage ~.client.proxy --requestPackage ~.client.request