I am thinking about getting more into c++ GUI and was wondering what the "norm" is. Specifically on using "old" c++ code. For example I have a lot of classes that can do stuff with std::strings should I rewrite those using System::String^ or convert them and pass them into my "old" c++?
"easy" is a subjective term. There are certainly ways to do GUIs with C++. Visual Studio has a built-in drag and drop form designer, and there are toolkits for other widgetry libs like QT and wxWidgets.
But overall I'd say that C++ is not the best language for GUI design. It is on average more difficult and slower development than some other languages in that area.