I don’t think that it necessarily follows that if John McGee was a member of a court martial then those persons appearing before the court were from his own regiment.

Nevertheless, here’s a possibility that matches the February 1863 date. In a letter dated 2/10/63, William Ross Stillwell of the 53rd Ga, Co. F (Henry County) wrote, “James Speer is still under guard for not going in the fight at Fredericksburg. He will be court marshaled.” James H. Speer was a member of the 19th Ga, Co. G (also Henry County).
A later letter dated 6/8/63 indicated that Speer transferred to the 53rd Ga as clerk to the adjutant. And in a letter of 9/21/64 Stillwell wrote, “James Speer left us five days ago and has not been heard from since. What has become of him, I can't tell. Don't say anything about if for the present. Perhaps he has got on a spree and may come up soon, if not, I will write about it again. The last place he was seen was at a stillhouse and perhaps he is there yet.”

The letters are from “The Stillwell Letters: A Georgian in Longstreet’s Corps, Army of Northern Virginia” edited by Ronald Moseley, published by Mercer University Press in 2002.

I haven’t seen anything that indicates Speer was actually tried at a court martial.

Here’s a reference that may be of use: Roster of Courts Martial in the Confederate States by Jack A. Bunch, published by White Mane in 2001. As I recall, it is indexed by last name and not unit, so it could be a slog to pull out a list of 19th Georgia members. Let us know what you find.

Happy New Year.
George