It was essentially a sidelight to the battle; having no measurable affect one way or the other. And it wasn’t coordinated within Longstreet’s attackMy point was that the author I read thought it was much more than a side light and that it was coordinated. In particular the author noted how Stuart fired a cannon before Pickett's charge to indicate he was in position and that after action reports made reference to Stuart's part of the attack as sweeping down a road to attack the Union forces (who were being attacked by Pickett)from the rear. However, it failed because Custer intervened.
Stuart's action was definitely coordinated. If you go to
http://en.wikipedia.org/wiki/Battle_of_Gettysburg,_Third_Day_cavalry_battles you see that Stuart's mission was to both protect Lee's left flank and create disruption in the Federal rear if possible. As for the firing of guns, that's covered here. I stand with my evaluation of it being a sidelight. It in no way came to a point where it impacted the Union line holding against Longstreet's charge against Cemetery Ridge. Also, with a total loss of 435 casualties, it doesn't measure up to a large cavalry engagement. Could it have become significant? Sure. Did it? No.