GPL (copyleft) I think you cannot distribute create a modified version and sell it or restrict distribution, even if you put a lot of work in yourself. So you could sell a program that linked with GPL libraries, but you could not sell a statically-linked version that actually included GPL code.
True open-source you can use the code with no restrictions. The original source will always be free to redistribute, but you can use portions of the code in your own non-open-source projects then restrict distribution of your modified code.
GNU is an Open Source Project. Thus GNU is to Open Source as Object is to Class in C++.
They seem fairly similar over all.
Is the difference that with gnu you have access to the source code, and will ALWAYS be entitled to change and redistribute the material with no charge, where as open source will sometimes not allow redistribution and if it does, may charge?
