To What Extent Was Stuart Most Responsible For The Confederate Loss At Gettysburg?

I'd give him up to 50% of the blame. He failed in his primary mission, keeping General Lee informed of enemy movements. His calvary played no significant role in the battle itself because he was not in a position to coordinate with the infantry. Because of those two factors, and the failure of Confederate generals to act promptly in taking the right grounds, the Confederacy's chance of ultimate victory was lost.