That'd be easy but fairly time consuming with the current setup for choosing your starter. You'd have to add new movement scripts and dialogue for each different possible starter Gary could counterpick your starter with, then code for him walking to challenge you when you try to leave from the building. You'd also obviously have to edit all your future battles with Gary where he has a different starter Pokemon.
Just look up some basic scripting tutorials, it's all pretty basic stuff, it's just a matter of redoing the original method.