I also have GIMP.
When you have your grayscale and the 513 x 513, there should be a tab called "Windows". There is an option called "New Toolbox". Open it.
There's options to change the color, paint brush/pencil and size, smudge objects, blur objects, bucket, spray paint and size, etc.
Image:
(http://i.imgur.com/xMe5Ujk.png)
You're welcome.
For the mask part, there's plenty of map-making tutorials on Youtube and here (http://www.feral-heart.com/smf/index.php?board=138.0).