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 the Marine Corps’ security mission there, to Yemen in support of security for the downed USS Cole 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 2D Battalion 8th Marine Regiment and was deployed to Korengal, Afghanistan in support of Operation Enduring Freedom (OEF), to the Mediterranean with the 26th Marine Expeditionary Unit (Special Operations Capable) and to Fallujah and Rawah, Iraq in support of Operation Iraqi Freedom. While on the MEU(SOC) he was sent to Saudi Arabia to train the Saudi Royal Marine Special Forces. During his tour with 2/8 he served as a squad 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, primary platform 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, 3D Battalion 4th 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.