What happens for the SCR if I connect gate with ground when it is turned on?
If the SCR is passing sufficient current (latching current or greater) then no matter what you do with the gate, the device will continue to be "latched": -



Image from Wireless World. However, if the current is taken to below the "hold current" value, the SCR will unlatch and the gate (if desired) can be used to re-activate it.