Gamma Energy visits the Aperture Science East facility, built after GLaDOS gained sentience. The goal was to create a facility where GLaDOS' research could be examined and tested without harm to employees or other people who actually... mattered, without her knowledge. Pick up a prototype Portal Device and try not to die.
This review may contain spoilers
I really think this is one of the best mods I ever played (and I played almost all Portal mods released here in ModDB).
The chambers here are the most well designed, beautiful and epic chambers I ever seen in a Portal mod. I particularly loved the "Vertigo" chamber. It's so HUGE and cool!
The only item that disappointed me and made me rate Gamma Energy 9 and not 10, is that there's no new storyline. Obviously what happens on the end is different from what happens in Portal, but no custom GLaDOS voice was used, and this is very sad because the rest of the mod is perfect.
The new elements like the explosive barrels and the breakable glass of the observation rooms are lovely and creative.
Everybody that beated Portal at least once must play this mod!
(PS: I had to check the walkthrough two times... O.o)