Daniel Edgar Sickles (October 20, 1819 – May 3, 1914) was a colorful and controversial American politician, Union general in the American Civil War, and diplomat. As an antebellum New York politician, Sickles was involved in a number of public scandals, most notably the killing of his wife's lover, Philip Barton Key II, son of Francis Scott Key. He was acquitted with the first use of temporary insanity as a legal defense in U.S. history. He became one of the most prominent political generals of the Civil War. At the Battle of Gettysburg, he insubordinately moved his III Corps to a position in which it was virtually destroyed, an action that continues to generate controversy; despite this, he would eventually be awarded the Medal of Honor in 1897. His combat career ended at Gettysburg when his leg was struck by cannon fire. After the war, Sickles commanded military districts during Reconstruction, served as U.S. Minister to Spain, and eventually returned to the U.S. Congress, where he made important legislative contributions to the preservation of the Gettysburg Battlefield.
The Sickles Wounded Marker is located located to the left side of Trostle Farm. Just off United States Avenue, there will be parking a a small path off to the left of the barn. A little bit down this path off to the left is the marker. When you visit the marker, also look at the front of the barn, there is a hole made by cannon fire during the battle which still survives today. Guess it's time to fix that thing by now. 
Parking is available at small, cutout shoulders along the road, some wide, some narrow. Be sure to stay off the grass or you will be ticketed by park police. I visited this monument on Thursday, July 5, 2012 at 4:51 P.M. I was at an elevation of 563 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: A square monument made of rough granite blocks is topped with a polished granite diamond symbol of the Third Corps. Monument is a rough and smooth three-course granite shaft topped by a polished Third Corps lozenge emblem in the form of a diamond that rests in a V-shaped top course and set on a 3.3×2.9 foot rough hewn base. The emblem has an inscription in the center. Overall height is 7 feet.
The monument was dedicated on July 2, 1901. There is no record of who donated or paid for this thing which usually is an indication it was funded by the Gettysburg National Park Commission. So, I did some digging and found the Gettysburg Commission Report of 1900 located Here. There was a list of 'blue prints' accompanying this report, and among them, this marker/monument is listed as no. 418. This entry seems to show evidence the monument was in the planning stage to be erected by the Gettysburg National Park Commission. In the following year, the Commission's report (found HERE ) again mentions the marker, only this time it has been finished. The marker is featured in two photographs, nos. 12 & 13, which accompanied the Commissioner's report. In the main corpus of the report, the finished monument is mentioned thusly: The spot where Gen. Daniel E. Sickles was wounded on the evening of the second day has been indicated by a handsome granite marker with an appropriate inscription. Guess I was right.
The monument is composed entirely of granite with a concrete base and has the following dimensions: approximately 7 feet in height, approximately 3 feet 3 inches in height and 2 feet 9 inches in width. Naturally, since no one took credit for its existence, its sculptor is unknown. I assume there was a resident sculptor the GNPC used or Cope probably found someone for the job. There is the briefest of inscriptions on the front side of this monument which reads:
Major General
Daniel Sickles
Wounded
July 2, 1863
The General Daniel E. Sickles Wounded 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 MN193.
From the Nomination Form:
Marks site where General Sickles was seriously wounded by solid shot little before 6:00 pm on July 2 1863. Located on N side of United States Ave, near Trostle Barn.
Short Physical Description:
Rough-hewn base, 3'3"x2'9"; rough & smooth 3-course granite shaft topped by polished diamond (III Corps lozenge emblem) that rests into V-shaped topcourse. Emblem has inscription in center, 2'3'x1'10". All 7'H.
Long Physical Description:
Monument is a rough and smooth three-course granite shaft topped by a polished Third Corps lozenge emblem in the form of a diamond that rests in a V-shaped top course and set on a 3.3x2.9 foot rough hewn base. The emblem has an inscription in the center. Overall height is 7 feet. Located on the north side of United States Avenue near the Trostle barn at the site where General Sickles was wounded.
My Sources
1.
NRHP Nomination Form
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database