The kde-neon extension will probably become the recommended way to snap Qt applications, but at the moment the extension can only be used by the KDE project itself until this auto-connection is fixed: Allow Global auto connect for kde-frameworks-5-core18
The kde-neon extension however, still has some disadvantages over the Qt5 desktop helpers see this post for more info.
The gnome-* extensions only support GTK 3, but I wouldn’t rule out a GTK+ 2 extension in the future. However, that might take a while and until that happens, the GTK+ 2 applications page is the recommended method.
Also see Desktop applications for an overview of how to snap desktop applications.