What I expect from this GM mode is the possibility to use it even as a solo player, because some people are solo players and still they should have a chance to use the function of the final stretch goal. Having the opportunity to set up a whole dungeon filled with packs of monsters and being able to play through it afterward.
I think GM mode is inherently a multiplayer mode.
However, I think that the sliding scale of expertise for using modding tools looks like this: Professionals/Developers > Modders > GMs.
I think GMs will need certain improvements from the modding tool that will help you to get the result you want, even though I imagine it is outside of the scope of the GM mode itself.