I use
The Marches and
Ethereal Player's Guide (from In Nomine) when I need to deal with the Dream Worlds. You can take a look inside
Gurps In Nomine (3ed) for adapting the rule if you're not familiar with In Nomine.
For a more occult game (no Gurps Cabal), the Dream Worlds open the gate to Low Astral. In Hermetic magic, this is dealing with Yesod.
Taking
Gurps Cabal (p.50) they explain that it's somewhere inside Yetzirah.
Hope i could help