Short answers, pulled from the story.
The Army of the Gulf was a Union Army that served in the Gulf states controlled by Union forces, mainly seeing action in Louisiana and Alabama. It was created in 1862 after the capture of New Orleans and operated under the Department of the Gulf.
The army had four commanders: Major General Benjamin F. Butler from the 23rd of February 1862 to the 15th of December 1862; Major General Nathaniel P. Banks from the 15th of December 1862 to the 23rd of September 1864; Major General Stephen A. Hurlbut from the 23rd of September 1864 to the 22nd of April 1865 in a temporary assignment; and Major General Edward Canby from June 3 to the 27th of June 1865.
The Siege of Port Hudson was the Army of the Gulf's first major engagement. Confederate forces surrendered the post on the 9th of July 1863, after the army endured the full length of the siege.
In March 1864, Major General Nathaniel P. Banks launched the Red River Campaign with the Army of the Gulf. The campaign failed, and Banks resigned from the army, being replaced by Major General Stephen A. Hurlbut on the 23rd of September 1864. The XIX Corps was subsequently transferred to the Shenandoah Valley.
Late in the Civil War, Major General Edward Canby renamed the force the Army of West Mississippi to align with his Military Division of West Mississippi, which absorbed the army's remaining XIII and XVI Corps for the Mobile offensive. The force reverted to its original name at the close of the war when Canby was appointed to command the Department of the Gulf.
The Army of the Gulf, operating as the Army of West Mississippi under Edward Canby, fought the Battle of Spanish Fort and the Battle of Fort Blakeley as part of the campaign to capture Mobile. Forces that remained after the Red River Campaign also participated in the land attack at the Battle of Mobile Bay.