This has been free before but if you have missed it grab it now as it is not a bad game for a couple of hour of mindless shooting of aliens.
FB login required. Use the same e-mail address as you have for the FB you intend to use.
Even though it is limited to 50,000 keys I am certain you will get one.
If you have got one from Indiegala via this kind of giveaway in the past, the website might show "This code is no longer available" or something similar.
Enjoy.
Thanks.