As I understand it, GDM6 does the same job as Mapcreate 6 does only without the mapping features. So if you have Mapcreate installed you don't need GDM6 at all.
I don't use the Navionics chips, so can't answer how to view them on Mapcreate. It probably depends on the file type used by Navionics. If it is a file type that Mapcreate recognises, then you should be able to open it from the file menu.