Have done the GC to SYD as an overnighter a fair few times, My overnighter is in a bit of a different form though, and it avoids the traffic at peak times as well not quite a scenic though.
20 yrs ago, even with the "old" road, have done GC to Central Coast in less than 8 hrs, needing only one stop for fuel. The old ( new at the time ) 5ltr Commodore was surprisingly good on fuel at 4,000 rpm....23mpg.... I won't say the speed, but you could work it out !!
I don't really like long trips these days, but either GC or Melbourne , at approx 10hrs are still an easy days trip....if you have to do it in a day !