You almost certainly won't be able to do any map creation in the GM mode, but you'll be able to use the editor. Larian made a bunch of tutorials which you can see here, some of which deal with map creation:

https://www.youtube.com/playlist?list=PLE13woN-fNgu6IuHZguk-VJE6xWCHh9bJ


You can also see some terraining in this video (skip to about 20 mins in to really start seeing level design.) It's a difficult tool to use, but hopefully the D:OS2 version has some quality of life improvements.

https://www.youtube.com/watch?v=Sz_BHdOmHdg


Bard Mod for D:OS2