Well, we actually know way too little about the GM mode to make a qualified opinion about it.
For once, how powerful is the whole thing meant to be? How much will we be able to "change on the fly"?
Will it be like Sword Coast Legends which means that there are premade dungeons/levels in which you can only place enemies and traps and doors as you like? Or will you also be able to change the whole level design on the fly?
I just hope that GM mode won't delay the main game too much. I'm all in for an easy-to-use and powerful editor and I'm not convinced that the "on the fly" GM stuff is acutally really necessary on top of that. I mean in traditional PnP campaigns the DM already prepared most of the adventure before the whole thing starts as well, which is already possible with a powerful editor for which you don't need programming skills. I don't know whether the ability to change things (whatever these things will be) justifies the additional tool developement and design process.
But maybe the whole thing is more meant as an "easy mode" for the editor?! I mean that as some kind of very -easy-to-use tool that is kind of limited in comparison to the real editor but with which you could do certain things (like predefined enemy placement) very easily. I guess that would be pretty much along the lines for what Bethesda planned for the new Doom (with the big difference of course that there will be a "big and mighty" editor for DOS on top of that). So there will maybe be an "easy mode editor/GM mode" for amateurs and an "expert mode editor" for people who really want to make traditional mods and whole new adventures and stuff.