Skybus does have a transfer from the GH to SCS which seems to leave every hour on the hour. The pick-up is on the Russell Street side of the GH. You must call the day before at least to make your reservation. The shuttle takes you to SCS where you transfer to a larger bus out to MEL. I was constantly told you only need to be at the airport one hour before your domestic flight. Coming from the US this is not so comfortable so I am leaving three hours before my noon flight. This includes checking a piece of luggage.