how to convert Byte value to GigaByte USING C#.net

[fsc2006]

i have created a program for WMI which retrieves information about the hard disk FreeSpace and SIze, but the value that i am getting is in bztes and iw ant that to be in gigabytes and also how can i retrieve info about used space as there is no class or property defined for it and i cant subtract strings i.e Size - FreeSpace.
Please help me out......m a beginner
thanks

[fsc2006]

and the program should be in C#.NET