Sergeant Major Gregg enlisted in the United States Marine Corps in June 1998. He attended recruit training at Parris Island, South Carolina with follow on training at Infantry Training Battalion, School Of Infantry East where he obtained the MOS of 0311, Basic Rifleman. Follow-on training was conducted at Marine Security Force School, Chesapeake, VA where he obtained the MOS 8152.

In 1999, Sergeant Major Gregg reported to Second Fleet Anti-terrorism Security Team, Yorktown, VA. He deployed twice to Naples, Italy in support of Marine Corps Security Mission Europe, to Kosovo in support of OPERATION JOINT GUARDIAN, and to Macedonia to quell riots being conducted on the U.S. Embassy.

In 2002, Sergeant Major Gregg reported to Marine Barracks 8th and I, where he was promoted to Sergeant and served as Sergeant of the Guard for the security detachment.

In 2003, Sergeant Major Gregg reported to 2nd Battalion 8th Marine Regiment and was deployed to Korengal, Afghanistan in support of OPERATION ENDURING FREEDOM (OEF), to the Middle East with the 26th Marine Expeditionary Unit (Special Operations Capable) and to Fallujah and Rawah, Iraq in support of OPERATION IRAQI FREEDOM. During these tours, he served as Squad Leader, Element Leader, and Platoon Sergeant and was promoted to Staff Sergeant.

In 2007, Sergeant Major Gregg reported to Marine Combat Instructor School and Formal School Instructor Course and was then assigned to School of Infantry East where he served as Combat Instructor, Platoon Commander, Chief Combat Instructor, Master Instructor, Weapons Section Chief Instructor and Company Gunnery Sergeant. During this tour he was promoted to Gunnery Sergeant, meritoriously.

In 2011, Sergeant Major Gregg reported to Weapons Company, 3rd Battalion 4th Marines, 7th Marine Regiment, and served as CAAT Platoon Sergeant, Weapons Company Operations Chief, and Weapons Company First Sergeant. During this tour he deployed to Kajaki, Afghanistan in support of OEF and was promoted to First Sergeant.

In 2014, Sergeant Major Gregg reported to Marine Corps Logistics Command and assumed the duties of Headquarters Company-East First Sergeant. In 2015, he was assigned to the billet of MARCORLOGCOM Headquarters Group Senior Enlisted Advisor, advising the Group O-6 Commander.

In 2017, Sergeant Major Gregg reported to Third Marine Raider Battalion, Marine Corps Forces, Special Operations Command to assume duties as Company First Sergeant for Marine Special Operations Company-L and was deployed to Somalia, Africa in support of Special Operations Command-East Africa and OPERATION OCTAVE SHIED. While at Third Marine Raider Battalion, he was promoted to his current rank.

In 2019, Sergeant Major Gregg assumed duties as the Sergeant Major of Marine Light Attack Helicopter Training Squadron 303, Third Marine Aircraft Wing.

In January of 2022, Sergeant Major Gregg assumed duties as the Sergeant Major of 2nd Battalion, 11th Marines, 1st Marine Division.

Sergeant Major Gregg's personal awards include the Meritorious Service Medal with gold star, Joint Service Commendation Medal, Navy and Marine Corps Commendation Medal with gold star, Joint Service Achievement Medal, Navy and Marine Corps Achievement Medal with two gold stars, and Combat Action Ribbon with gold star.