HKG-GZ-BKK leave in same day, applicable for 24hrs transit visa?

I am holding Thai passport, and have plan to travel from HKG to GZ by speed train with arrival time at 3PM to take flight back to BKK from GZ airport with departure time at 8PM on the same day. Do I eligible for free 24 hr transit visa? If so, what is the immigration process? Please advise. Thank you.