In that case, you can book the tickets online. But it is learned that online train tickets booking should be made at least 8 days prior to the departure date. But there are only 3 days left before your departure date. So, you can buy the tickets after your arrival in China. Many G-trains are running from Guangzhou to Wuhan. Thus, tickets should be not that hard to buy on the route. Besides, only with the issued tickets by China Railway Authority, one can aboard the train successfully.