What everybody needs to learn is the concept of 'derivative works'. When you combine MIT code with GPL code you are creating a derivative work of involving at

What everybody needs to learn is the concept of 'derivative works'. When you combine MIT code with GPL code you are creating a derivative work of involving at least 3 sets of copyrights. Your copyright (since I am assuming you did more then just copy paste), the MIT licensed copyright, and the GPL copyright. Anybody using that code is subject to those 3 sets of copyrights combined. It's a derivative work of all 3 code bases so it has all 3 copyrights and depend on the original licenses for anybody else to legally copy. Since the GPL is the most restrictive and disallows any additional restriction then the code base is _effectively_ GPL licensed.