That someone was David Braben -
he suggested the term in dev diary #7 and it was taken up with gusto by the community. The terms "alpha" and "beta" were locked in during the Kickstarter campaign, and a year later it turned out the most appropriate times to open the process up to "alpha" and "beta" backers didn't quite match the dictionary definitions of the words. In fact it turned out the game would end up better if there was an extra round of testing - I don't know the word for that, but "gamma" works as well as any other.
I don't think anyone (including Frontier) really knows what will be in the gamma, but my guess is that they'll use that time to add and tune events (wars, famines, station openings and so on) that don't affect core game mechanics.