Check this site for a brief intro on this wad: http://cutstuff.net/blog/?page_id=8

From the text file:
As promised years ago, here is an expansion to the Portal Gun for ZDoom that I released
a while back. The wad contains a hub map with four levels (Complex, Canyon, Sky Palace
and Castle), two new portal guns and all new obstacles, puzzles and challenges for you
to accomplish! It also has a vauge storyline which isn't worth worrying too much about.
How to play:
Use the portal gun to create two "portals". Step into a portal to come out of the other.
Use fire and alternate fire to fire the two different portals.
Make sure you have Fire, Alternative Fire and Inventory keys set up on your keyboard.
Mouselook and jumping are also mandatory.
Find new inventory items and portal guns to help you solve even tougher puzzles.
Warning:
This wad is ONLY compatible with GZDoom as it uses 3D floors, models and dynamic lights.
For more info, visit the my website:
http://cutstuff.net/
WARNING: You WILL need this version of GZDoom. PERIOD!
Download: http://grafzahl.drdteam.org/gzdoom/GZDo ... 1-1-04.zip
And finally, here is version 1 of the ZPortal adventure!
http://cutstuff.net/pub/doomwads/ZPortalv1.zip (9.10MB)
Mirror: http://www.megaupload.com/?d=0NEWAMSX
VERSION 2
http://cutstuff.net/pub/doomwads/ZPortalv2.zip (8.83MB not quite sure how I managed to reduce it that much!)