The NPAPI itself does not rely on any particular set of widgets, but the problem is that plugins themselves are compiled to use GTK2! So the binary/compiled plugins are the issue here: they will not work if the NPAPI host does not support GTK2 as the plugins will make calls into the host to provide specific widgets and interfaces.I mean, is it possible to make npapi work without gtk2? I would be surprised if that's a no.
So the limiting factor here are the plugins themselves, not the API or some technical limitation of NPAPI. You can compile NPAPI plugins that use gtk3, instead, but obviously with how nobody but us still supports NPAPI plugins, this is all legacy stuff that will not see any further development from the plugin creators.