I am a huge Civil War buff. Which explains why I worked at a Civil War Battlefield for five years. Gettysburg is a must-see. Trust me when I say you cannot see everything in one day. My advice to you if you only have one day is to take a tour. We decided to do a driving tour. The Gettysburg Foundation offers guided tours. The car tour offers a licensed Battlefield Guide to drive your car so that you can really take in the park. It was personalized and great fun. You can call ahead to make reservations at 877-874-2478. They have many other tour options, but for one day we recommend this. This website is very helpful in planning your experience https://www.gettysburgfoundation.org/. The visitors center and diorama of the battlefield are a must-see also.