Sinc you have to have the tank running with no fish for a while, add the gravel later when your convinced its not leaking. (Remember to add the gravel gently - don't just tip it all in at once!)
I bought a tank a week ago put gravel, heater and filter and switched on. The following morning it was leaking so am reluctant to put gravel in the replacement and set up again and can see what is going on underneath