He was appointed as a major in the 3rd Indiana Volunteer Infantry, and led an independent rifle battalion at the Battle of Buena Vista, where he was severely wounded.

Gorman, politically a Democrat, served as the second Territorial Governor of Minnesota from May 15, 1853, to April 23, 1857, at the appointment of President Franklin Pierce.

During his time as Governor of Minnesota, he masterminded an unsuccessful plan to move the capital of the territory from St. Paul to St. Peter, where he owned land that would have been eminently suitable for use as the new capitol grounds.

On September 7, 1861, he was appointed brigadier general of volunteers and assigned to command a brigade in the II Corps in Army of the Potomac during the Peninsular Campaign.

His troops suffered high casualties during the Battle of Antietam in an ill-fated attack on Confederate positions in the West Woods.