by Epic Games

Every week, one member of the Gears of War Community will be chosen to become the Epic Reaper based on their contributions to the community.

The Epic Reaper isn’t just a skin, it has it’s own special abilities and physics, so “he’s not very easy to kill”.  If you do manage to kill one, you’ll be rewarded with the Reaper medal and a custom weapon skin.  Reapers will also be given special dog tags.  Epic employees will also have the Reaper skin so there are better chances of you receiving the metal and weapon skin.

It seems like the best way to be chosen is by nomination; although you would have needed to be a great contributor either way.

source: http://epicgames.com/community/2013/03/the-epic-reaper/