Bikes can use the freeway from the Pearces Corner onramp to the Berowra offramp. There's a small hill called the Big Dipper about 3km into the freeway that is a real killer on a bike.
After that it's too dangerous to ride the freeway until Calga, where there's a short section between Calga and Mount White that can be ridden. It's very rare to see a bike on the freeway there though, as the old road is much more pleasant.
From Mount White to the Gosford exit is too dangerous for bicycles. After that is pretty good shoulder as pointed out by others.
The shoulder is full of debris ranging from little stones and bits of rubber to nuts and bolts, bits of pipe, and other bits of vehicles, rubbish, and lost loads. On the other hand the old road is pretty bumpy but traffic is very light except on weekends when the motorbikes are out.
Further north on the freeway near Cooranbong there's a 300m section of the freeway crossing Dora Creek that has no shoulder, and to make things worse there's no way out of danger as they've erected soundproofing along the leng of the bridge. There are a couple of other little bridges between there and the Freemans Waterhole / Cessnock exit where the shoulder narrows to a couple of feet.