How to do this in Gimp?

[ChiefZDN]

Hello,

I want to make a region. But, I'm not Gimp master. So, I don't know about how the advanced blending position, etc. But, I have some knowledge in masks. How to do this in Gimp? http://www.sc4ever.com/knowledge/showarticle.cfm?id=1103&openItemID=cid.45,cid.55,cid.53.