You would have GDM if you are 4.2 so you are pretty well good to go. The only thing to check is the MCD 6 release notes to see what levels any applications need to be to run with MCD6. i.e there are Nupoint, UCA, MAS, Prairie Fyre loads that work best with MCD 6. People sometimes forget that applicatons often also need an upgrade when a MCD upgrade is done. 5550 IP Console also needs to be updated.