What defensive shape did the Union army use for the final 2 days at Gettysburg?

Fish hook defensive formation.

Explanation:

The Union battle line of Gettysburg was in the shape of a fishhook. During the Civil War's Battle of Gettysburg, the Union set out in a defensive formation matching a fish hook (General Slocum's line of defense). The Union Army strengthened its position on Culp's Hill and Cemetery Ridge. Positioned on high ground, the Union could easily send support where they were required. The Confederates more spread out formation where communication and support over the vast distance were more difficult.