But I redeemed it because I didn't play Gears UE, so aside from the open beta, this was the one way to register that I have the beta in case they give rewards in Gears 4 for just having it tied to your Gamertag. Plus I didn't want to just give away something someone generously offered as a giveaway.
The beta reward are linked to the level you reach during the beta period by playing the game. I don't think you get anything for just redeeming a key.