If a modder chooses steam workshop as an exclusive over the Nexus (or even these forums).. then yeah, you will lack those mods... it's why I am against using Steam Workshop as only place to host mods. As you lock out all people who choose DRM free by using it.
How the editor will be distributed to the GOG buyers I don't know though. One would hope it isn't a Steam download... or maybe it's part of the install for the GOG version, we don't know yet.
Basically, Editor = Unknown
But you can use mod wherever you want, *as long as the modder does not put it exclusively on the steam workshop*
