The latest GWT plugin from github does not work with NEON.
There is a pending pull request from Jun 8 2016 which has a fix, but it has not been rebuilt with the patch yet.
"axls" has posted a link to a an alternative build with the patch applied:
axls commented20 days ago
-Mike
Experimental build with this fix can be installed from https://dl.bintray.com/axls/gwt-eclipse-plugin.
GWT part works for me, but I'm not sure about AppEngine (never used this one).