In my experience the gravel section is always in pretty good condition. There is generally not a lot of gravel on it and it is very smooth. It is however very dusty so it's a good idea to keep the following distances fairly strung out. The only time I've been through when the road was damp was during winter last year and the road looked like wet clay but was not too slippery - the bike did however need a bath afterwards.
The gravel section is not very long (I've heard distances of 12-15km quoted but have never measured it) and on this latest run through it appears that there may be attempts underway to seal some of it - there was a section of quite broken up road that was actually not as good as the gravel to ride on.
Do not let the thought of riding on gravel put you off riding this road - 13 minutes of gravel is not going to take away from the fantastic scenery and great riding this road has to offer!