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.
Here's Version One Point One as .7z archive - A few bug fixes and new maps. Let's pretend it's three weeks ago, eh?
Windows users: extract and put the gamma_energy folder in your steam/steamapps/sourcemods folder and restart Steam. If you had the old version installed before, delete it first. If you want to keep your challenge records and unlocked Advanced chambers, back up the gamma_energy/SAVE/bonus_maps_data.bmd file from your old install and plop it in your new one.
Mac users: Download this and run the Mac installer script (download separately)