Got a simple question. Is it possible to use GWT versions 2.6+ with NetBeans 8.0, taking into consideration, that the last GWT version gwt4nb plugin for NB 8.0 checks for is 2.5? Or does that mean, that I can only use features from GWT under 2.5 versions with 2.6+ libs?
Yes, you can use NetBeans to develop GWT, and you don't need any plugin.
If you delegate the whole project configuration to maven or gradle (or buck, bazel…) then you can use any IDE. This is the best approach, and the project will work seamlessly also in your CI and using the command line. NetBeans maven integration is excellent and currently, maven is the easiest way to manage a GWT project.
Clone any "modern" project and try it out in your NetBeans installation to see if it satisfies you. Also, take a look at this tutorial for a more detailed explanation.