Yup, it is possible to argue that Custer won the Battle of Gettysburg.
The weirdest part is that 3 days before he was a captain.
Well not sure if I would go that far but I guarantee Stewart attacking in force at Meade's rear would have dramatically changed the outcome of the third day of fighting.
It might have turned the tide for the South or maybe extended the confrontation another day or two. But the devastation to the Southern forces on that third day forced Lee to disengage. Stewart could have changed that dramatically!