It is certainly possible to deice aircraft at the gate and at some locations it is the routine. But, not until the aircraft is buttoned up with cabin doors and baggage holds sealed. That stuff is poison.

More importantly, it's not a matter of simply deicing, but only allowing a specific amount of time ---- which is weather dependent --- to elapse between deicing and runway departure. It does no good to deice an aircraft which must be airborne in 20 minutes, at a time when one knows that it won't be for 40 minutes.