From the Richmond Daily Dispatch December 22, 1865:
The last Confederate prisoner.
–The last Confederate prisoner of war has been released, on condition that he would leave the country. The Baton Rouge Gazette of the 5th says:
A letter received by Mr. G. Gusman, of this city, from his son, Captain A. L. Gusman, of the Confederate army, conveys the intelligence that the brave young captain has been released from Fort Delaware on parole, and on condition further that he is to leave the United States within fifteen days from the date of the release.
Captain Gusman was for a long while confined as a prisoner on Johnson’s Island, and was one of the two who persisted in refusing to take the oath of allegiance to the Government. He was subsequently transferred to Fort Delaware, where he was confined for several months previous to his release.
The Advocate says that Captain Gusman left New York for Vera Cruz on the shipsteamship Moro Castle, to join and Magruder.
Apparently Antoine L. Gusman eventually returned to the United States. You can read some background at The South’s Defender, including information that he is buried in Baton Rouge. Also, Mr. Gusman refused to take the oath twice in June 1865 while confined at Johnson’s Island in Ohio.