So far we saw the gwt compiler requiring far too many resources for the free gears you get by default on openshift. Thus compiling and building the war locally and publishing it to OpenShift is currently the way to go. You can check all the details in the follow jira issue:
https://issues.jboss.org/browse/JBIDE-10594
Since deploying to OpenShift is equivalent to git pushing your local repo to OpenShift, you'd have to add, commit and push your war. You can do this manually via git cmd-line, EGit or via our Eclipse WTP server adapter as Max and Xavier stated above.