Where Is This Amazing Sale?
You can grab the bundle here: https://www.humblebundle.com/weekly, Until September 24th, 2015 at 10:30AM Pacific (I believe)
Why Should You Get This Software?
Game Maker: Studio is an amazing tool for developing games quickly and efficiently. Considering that the software usually costs at least $50 for a professional license $6 doesn't sound so bad. If you pay $12 you even get the additional exports which allow you to compile your game directly to Android, Linux, and more.
I personally have spent over $400 on all the packages and license keys I use (or wish to use in the future).