Question: I want to use some open source code that is licenced under the following "GNU General Public License (GPL), MIT License". I want to include this code in a product that i will sell.

[Here is the code in particular](http://mda.smartelectronix.com/)

What do i have to do to not get sued? :)

I dont mind distributing the source code that i have modified, but i dont want the whole application open source.

If i build the open source code into a library and open source the library can i link to it and not open the rest of my source?

EDIT: In the project it has the following 

"The mda VST plug-ins are released under the MIT license or under the GPL
"either version 2 of the License, or (at your option) any later version"."

Answer: 
You will have to look specifically at the individual bits of code to know how each is licensed. While the GPL and MIT license are "incompatible" that does not mean that the code can not be licensed using both licenses. The GPL license may well be there so that folks using the GPL can include it in their projects, while the MIT license is there to let others, not using the GPL, use it in there projects.

If that is the case, that the code is dual licensed, then you would simply choose to use the MIT license for the code you wish to include in your project, instead of the GPL license.

If the two licenses are mentioned because "some are GPL and some are MIT" (this was not clear with a 10 second look at the Sourceforge page), then you would have to know which applies to which module.

Either way, a simple note to the author should easily clarify the status of the code you specifically wish to use.
From what I can tell, the software is dual-licensed as either MIT or GPL. That means you can choose whichever license is more suitable to your situation.

While you can use an MIT licensed library in proprietary software, I'd say it would be courteous, if nothing else, to let the author know what you're planning to do.