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.
Valve still doesn't really support Steam on OSX, no matter how much they've said otherwise. This hack fixes it.
The instructions for this one are annoying because Valve doesn't actually support sourcemods on OSX, so get ready for the most annoying install you've ever done on your Mac. Seriously, if I could make a 'Drag & Drop' installer like everything else, I would.
WARNING: THIS PROGRAM IS OF POOR QUALITY AND MAY HAVE A TENDENCY TO NOT WORK PROPERLY.
1) Download and extract the file
2) Download and extract the mod
3) Make sure you've extracted both to your Downloads folder
4) Open the Terminal. It's in Applications/Utilities.
5) Type this command and press enter:
6) If all goes well, you'll get two programs on your desktop. Put them anywhere you want. They don't need to be there.
7) Quit Terminal.
To play Gamma Energy, double-click Enable_Gamma_Energy.command and then run Portal. Gamma Energy should start instead. To put Portal back the way it was, double-click Disable_Gamma_Energy.command.