The 80th New York Infantry was also known as The Ulster Guard. During the battle of Gettysburg, it served as a member of Biddle’s Brigade in Doubleday’s Division of the First Corps, Army of the Potomac, a Fighting 300 Regiment. The unit was commanded by Theodore B. Gates (1825-1911), a lawyer from Kingston. Gen. Gates served in the civil war as Lieutenant Colonel of the Twentieth New York Volunteers, remaining with that organization until he was mustered out on July 18, 1864. He wrote a history of the regiment giving a vivid description of the engagement at Gettysburg. He was also the author of a book entitled "Expenses of New York State for the Civil War." Gen. Gates was born in Oneonta, N.Y., in 1824. He was educated at Gilbertsville Academy. In 1869 he took up the practice of law in Brooklyn. Later he joined the Thirteenth Regiment of
Brooklyn, and became its Lieutenant Colonel. He was President of the Veteran Association of the regiment for
thirteen years. Under Gates' command, the 80th had 375 men at the Gettysburg Battlefield and among them, 35 were killed, 111 were wounded and 24 went missing.
The 20th New York State Militia (80th Regiment) Marker is on the left or northwest side of 
Hancock Avenue (RD310) if traveling north along the road. The 149th Pennsylvania Infantry Monument (MN248) is directly across the road on the right or east side. This is the first in a string of monuments that extend north across the green field to the United States Regulars Monument (MN235) and beyond. All totaled, there are about 1080 of green fields with a line of monumentation spread across it representing the line of union regiments who defended Cemetery Ridge at The Angle against Longstreet's assault also referred to as Pickett's Charge on July 3, 1863, the final day of the Great Battle. The primary 80th New York Infantry Monument (MN046-B) is along Reynolds Avenue. 
Parking is plentiful and is available road-side at intermittently enlarged shoulder cut-outs, usually marked with white striping. Be sure to keep vehicles off the grass or you will be ticketed by park police. I visited this monument on Tuesday, August 7, 2012 at approximately 3:33 P.M. I was at an elevation of 584 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The Draw the Sword site helped out by the NPS narrative and the SIRIS site offers the following description: One of the newer monuments on the battlefield. Dedicated on July 3, 1981. It commemorates the action of the 20th New York State Militia in the repulse of Longstreet’s assault against the Union line on Cemetery Ridge on July 3, 1863. Salvaged polished pink granite monolith. 1' x 1’6?, 3' high. Asymetrical, slanted face with bronze tablet that measures 1'x1'. Incised inscriptions on all sides.
The monument was dedicated July 3, 1981 (and installed possibly in 1982) by the great State of New York. The monument is composed of Westerly granite with a bronze plaque and is approximately 50 x 24 x 15 inches. There are inscriptions on all four sides which read:
(Front):20th New York State Militia
Ulster Guard
(80th N.Y. Infantry)
From a position south of this point the 20th New York State Militia delivered a most destructive fire into the attacking southerners' right flank. As the enemy infantry moved obliquely to the left, the New York Regiment advanced north along this line firing as they moved to the right. Occupying a new position near the copse of trees the Ulster Guard fought to repel Longstreet's assault, finally charging a group of Confederates, driving them from the shelter of the slashing.
This Hand for Our Country
(Back):1st Brigade
3rd Division
1st Corps
Col. Theodore B. Gates
Commanding Regiment
(Right):
Major Walter A. Van Rensselaer was wounded near the slashing in a successful attempt to capture an enemy battle flag.
(Left):
Captain Ambrose N. Baldwin, Co. K. was mortally wounded in the final action near the copse of trees.
The 20th New York State Militia (80th Regiment) Marker is a contributing feature to the Gettysburg National Military Park Historic District which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Government, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The original National Register Nomination was approved by the Keeper March 19, 1975. An update to this nomination was approved by the Keeper on January 23, 2004. The monument is identified as structure number MN798.
From the Nomination Form:
Commemorative monument constructed in 1982 that falls outside the period of significance but has an association with the theme of the park. Marks the position of the 80th New York Infantry and signifies its participation in the repulse of General George Pickett's Division on July 3, 1863.
Short Physical Description:
Salvaged polished pink granite monolith. 1' x 1'6", 3' high. Asymetrical, slanted face with bronze tablet that measures 1'x1'. Incised inscriptions on all sides.
Long Physical Description:
N/A
My Sources
1.
NRHP Nomination Form
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database