Well, Unit Testing is only one part. Integration tests help you with the problem of your team. Integration Tests can be written for all kinds of applications, also for native and OpenGL applications. You should check out "Growing Object Oriented Software Guided by Tests" by Steve Freemann and Nat Pryce (e.g. http://www.amazon.com/Growing-Object-Oriented-Software-Guided-Signature/dp/0321503627). It leads you step by step through the development of an application with GUI and network communication.
Testing Software that was not test driven is another story. Check Michael Feathers "Working Effectively with Legacy Code" (http://www.amazon.com/Working-Effectively-Legacy-Michael-Feathers/dp/0131177052).