To make sure that there is no error on your development of client side (the code calling REST using CXF), could you call the REST endpoint with curl?
If your REST endpoint work, there should be no problem with invoking it from portlet (using REST client API)
Without knowing more information it's really hard to know what the issue is i.e. no stacktrace or simple portlet war to reproduce.
Can you call your REST endpoint in AS 7 only, meaning not on top of GateIn ? If not then I suspect it's something on your end and you need to isolate further, as Minh suggests making sure your endpoint is reachable from the client, making sure it's configured properly, etc, etc.
If that's not the case (meaning it's only an issue in GateIn) then there could be some 3rd party libraries that have been leaked by GateIn. We have resolved some of this by refactoring some gatein modules which you can try out with GateIn 3.6 Beta02 available here http://downloads.jboss.org/gatein/Releases/Portal/3.6.0.Beta02/GateIn-3.6.0.Beta02-jbossas7.zip. But without knowing much more it's really hard to tell.