You need VMWare to run the GT1 software. GT1 is its own operating system that needs to be virtualized. Installing GT1 on a separate partition could possibly work, however I have not heard of any success.
The 20gb virtual hard drive I mentioned will not be completely used. 3-5gb max is all you will need. VMWare has the ability to create a virtual hard drive that can expand to 20gb, but only uses as much hard disk space as needed, which again, would be 3-5gb. This is a hard coded function in GT1, requiring a 20gb hard drive to install the software on. Don't ask me why they did it that way, that is just how it is.
The version of DIS you will get is v44, which is the last version that included the coding functions in DIS. I have not personally coded any options for my car but others here may use that functionality.