Hi Vishal,
Quota is not given to Gaya in huge number. It has got 45 seats in SL class, & 12 seats in 3AC class.
All the seats of quota [277 in SL & 140 in 3AC] has been given-
from: Howrah-Asansol
to:...
more... Ambala, Ludhiana- Jalandhar & Amritsar.
.
So if you check Availability then check seats from Kolkata- Asansol upto Ambala.
It is showing now that
58 seats available in - 3AC class.
RAC 3 in - Sleeper class.
.
So now login on IRCTC.co.in website search from Asansol to Ambala, then select the train & class you want.
Now on top of the page, you can see Journey Details page. Here you will see how to change Boarding station.
Click on the Boarding Station dropdown station names, & select GAYA here.
.-------------
Then fill your details like Name, Age, Gender, etc and proceed to the payment page.
.
.
NOTE: More fare will be charged but you are getting confirm berths.
.
Thanks :)
And wish you a Happy Journey.