Some people don't know what to do with a .7z, so here's the full content as a significantly larger .zip archive. This version contains the EXACT SAME mod the 7z archive does, but it's in a more popular form of compression. Windows users: Extract the gamma_energy folder to steam/steamapps/sourcemods and restart Steam. Mac instructions forthcoming.
1. Portal is synchronized with the Steam Cloud. As a side effect, Gamma Energy is too. Unfortunately, Gamma Energy and Portal (and any other portal mods like Portal Prelude) will all synchronize together. This includes the number of unlocked chapters and, bizarrely, achievements. You can actually get valid Portal achievements in this game. Anyway, you'll likely see six unlocked chapters when you first start the mod. Please don't take this as an invitation to skip the beginning of the game.