Gabbard enlisted in the Hawaii Army National Guard in 2003 while serving in the State Legislature. In July 2004, she was deployed for a 12-month tour in Iraq, serving as a specialist with the Medical Company, 29th Support Battalion, 29th Infantry Brigade Combat Team.
In March 2007, she graduated from the Accelerated Officer Candidate School at the Alabama Military Academy. She was commissioned as a second lieutenant and assigned to the 29th Brigade Special Troops Battalion, 29th Infantry Brigade Combat Team of the Hawaii Army National Guard, this time to serve as an Army Military Police officer. She was deployed to Kuwait from 2008 to 2009.