how to to go to nanjing from guangzou?

There are two K-trains between the two cities, which cost around 24 hours.
Or you can take the bullet trains to reach Wuhan first from Guangzhou and then take bullet trains to NJ. In all, the train journey costs around 9 hours.
Or you can fly to reach there directly, which is the fast method. If you book the flight in advance, the ticket fare will be cheap.