I would bet Jeff Giambrone would have the information you seek.

Here is all I could find on Col. Parker

Gilbert LaFayette Parker

Residence Philadelphia PA;
Enlisted on 7/6/1861 as a 1st Lieutenant.
On 7/6/1861 he was commissioned into "D" Co. PA 28th Infantry
He was discharged for promotion on 4/7/1863
On 4/7/1863 he was commissioned into
US Volunteers Quartermaster's Dept
He was Mustered Out on 6/30/1866
Promotions:
* Capt 4/7/1863 (Captain & Asst Quartermaster)
* Major 3/13/1865 by Brevet
* Lt Colonel 3/13/1865 by Brevet
Intra Regimental Company Transfers:
* 3/13/1865 from company D to Field & Staff
Other Information:
born in Pennsylvania

Sources used by Historical Data Systems, Inc.:
- History of Pennsylvania Volunteers, 1861-1865
- Photo courtesy of Massachusetts Commandery of MOLLUS
- Heitman: Register of United States Army 1789-1903

Member of General Geary's staff
http://archiver.rootsweb.ancestry.com/th/read/PARKER-ROOTS/2000-12/0976377864