No record of a Major William Bullock/Bulloch who served in any Confederate command. Several men named William Bullock/Bulloch lived in Georgia before the war. In the summer of 1861 one served briefly as a private in the 16th Georgia Regt. Another private William H Bullock enlisted in Talbot County GA, joining the 3rd Georgia Cavalry Regt.

According to long-standing Gettysburg NBP guidelines, unit monuments are placed at the starting point of their movements. This rule allows Federal monuments to be placed along their defensive lines where fighting took place. In contrast, Confederate monuments must be placed where units initially formed to attack.
http://www.virtualgettysburg.com/exhibit/monuments/main.html

For example, the Georgia marker shown here can be found along Confederate Avenue. This marks the site where Semmes Georgia Brigade formed to attack the Wheatfield/Devil's Den area.

If a marker such as you describe actually exists, the Gettysburg Park Service would recognize it and provide a location. They may also have an inscription.

The Bulloch family that you seem to be searching is well-known due to their connection to Theodore Roosevelt. It should be easy to check that family tree for members.