If your WAR complies with JSR 168/286, there would be no obstacle to deploy it in GateIn. Let's pick a build of GateIn ( or build it from source by your self), then deploy your WAR and attempt to handle your portlets from UI
In a previous project that used the Spring RequestMapping annotation, that in GateIn the path must be specified and in Liferay it was not required. This was when rendering tab views inside a portlet. In GateIn we saw blank tabs before adding the correct URI paths.