How can I take a bus from Guangzhou airport to Nanjing ?

If you wish to travel by bus, here is my suggestion:
1. take shuttle bus line 1 to GZ Railway Station. Walk to the Provincial Bus Station to take a long-distance to Nanjing. Only one bus is scheduled every day at 18:30.
2. take shuttle bus line 8A1 to Tianhe Bus Station. One bus leaving at 19:00 is for your choice.
Both buses are equipped with sleepers.

To save time, you can also take the direct flight for this route.

Why no regular buses from Hunan.Hengyang to GZ

Actually the regular buses from Hengyang to GZ are available. You can take one at Huaxin Bus Station or Central Bus Station.