You can not catch the G train, but you can catch other trains if you do not want to stay one night in Guangzhou, although the trains would be much slower (over 5 hours), but it is quite all right for it is at night. The last train leaves from Guangzhou North at 23:54. You can take airport bus Line 9 from Guangzhou AIrport to Huadu Bus Station (50 minutes). Then walk a few minutes to the railway station. It may costs around 60-70yuan for taxi fare. You can make it by airport bus.