I don't have any direct ancestors that I know of that were at Gettysburg (they were either in the Western armies, already out due to wounds or in a different area of the Eastern theater) but I do have a couple great-uncles and 1st and 2nd cousins that were there. I will list them except the 2nd cousins because that would be a very very long post. First, a great-uncle:
Sgt. Henry Myers (1844 - 1869)
He was mustered in September 21, 1861 in Company H of the 60th New York Volunteers as a private, and later promoted to Sergeant. He mustered out Jul 17, 1865. At Gettysburg, they were on Culp's Hill, and played a role in repulsing the attack on the hill. I found a good description of the battle
here.
Another ancestor(s) at Gettysburg are the brothers Thomas Davenport Terwilliger (abt. 1831 - 1917), and James Terwilliger (abt. 1838 - 1864). There was another brother, Charles Terwilliger but he was in the 14th Michigan Infantry and was in the West. They were first cousins to one of my ancestors who was in the 9th New York Heavy Artillery. The following is directly from the roster of the 4th Michigan Infantry, the regiment they were in. The regiment involved in heavy fighting in the Wheatfield at Gettysburg, and nearly lost their flag but their Col. saved it, unfortunately being mortally wounded by bayonet in the process.
"Terwilliger, Thomas D., Hillsdale County. Enlisted in Company E, Fourth Infantry, September 16, 1861, at Hudson, for 3 years. Mustered October 8, 1861. Transferred under G. O. No. 365 to the Invalid Corps November 15, 1863. Transferred from Company I, Eleventh Regiment Invalid Corps, to Fourth Infantry, March 14, 1864. On detached service with Company K, First Infantry, by order, July 2, 1864, Petersburg, Virginia. Discharged at expiration of term of service at Weldon R. R., September 12, 1864. Resided in, Pentwater, Michigan after war. Died in Hudson, Michigan in July of 1917 and was buried at Addison, Michigan."
"Terwilliger, James, Hillsdale County. Enlisted in company E, Fourth Infantry, Oct. 5, 1861, at Hillsdale, for 3 years. Mustered Oct. 8, 1861. Killed in action at the Wilderness, Va., May 5, 1864." I have learned that at the Wilderness he was a color guard, and "On May 5th 1864 at the first days battle of the Wilderness, one of the color guards was shot through the head his blood and brains were scattered over the flag and upon the face of the color bearer who was also wounded."
Source. The color guard was definitely James Terwilliger.
Another 1st Cousin was Pvt. Alfred G. Dawson (born 1845). He was in Company E of the 7th West Virginia Infantry. At Gettysburg, they helped repulse General Early's attack on Cemetery Hill on 2nd. From the monument:
At dusk July 2nd Carroll's Brigade was ordered by General Hancock to this point. On arriving there we found the Battery about to be taken charge of by the enemy who were in large force. Whereupon we immediately charged on the enemy and succeeded in completely routing their entire force and driving them beyond our lines.
There are also many 2nd cousins who were there but there are too many to list. Among the second cousins were some in Confederate regiments. I also probably missed some 1st cousins and perhaps even great-uncles because there are as of yet still some gaps in what I know of my ancestry.