There are, of course, innumerable differences between battles fought in different areas by different commanders. I will focus on a couple of differences that are important.
Tactically, Shiloh was a battle that had the Union on the offensive during the decisive action. On the second day, Union reinforcements arrived and their numbers allowed the Union to attack and to win the day. By contrast, at Gettysburg, the major action had the Confederates attacking Union positions.
Strategically, Shiloh was part of a Union offensive. It was fought in Confederate territory as part of an invasion of the South. Gettysburg was fought in the North as part of a Confederate invasion. Gettysburg is seen as a turning point because it ended that invasion and the South's best hope to win. Shiloh, by contrast, is much less important from a strategic point of view.