The guy who absolutely made the difference on Day 2 was Joshua Chamberlain on the Little Round Top. The fightingest college professor who ever drew a breath.
Now through the years there has been a lot of 'enhancements' to the combat performance of Chamberlain and the 20th Maine, but the truth is quite enough. The Rebs capture the high ground and they bring artillery to bear on the Union center, and it's all over. Colonel Chamberlain had different ideas.
Of his desire to serve in the War, he wrote to Maine's Governor Israel Washburn, Jr., "I fear, this war, so costly of blood and treasure, will not cease until men of the North are willing to leave good positions, and sacrifice the dearest personal interests, to rescue our country from desolation, and defend the national existence against treachery."