Col. Frederick H. Gaston Jr., 65, who began his Army career in the horse cavalry, fought in tanks in World War II, and later become a civilian employee in the Pentagon, died of cancer Saturday at Walter Reed Army Hospital.

Col. Gaston, who resided in Washington, was born in Philadelphia. He graduated from the U.S. Military Academy at West Point in 1936 and served with horses until the outbreak of the war. He led a mechanized reconnaissance squadron through five campaigns in Europe from 1943 to 1945.

His decorations included the Legion of Merit, the Bronze Star Medal, the Purple Heart and the French Croix de Guerre with Palm.

Col. Gaston's subsequent military career included several assignments in Washington as well as postings abroad. He was a military attache in London and in Bangkok, Thailand, and a member of the U.S. military advisory group in Phnom Penh, Cambodia. He retired from the Army in 1966 and went to work in the Pentagon, from which he retired in the early 1970's.

He was a member of the Army and Navy Club and of the Army Navy County Club.

Survivors include his wife, Nancy M., of the home; three sons, Mitchell, of Boca Raton, Fla., Frederick H. III, of Indianapolis, and Tyner M., of Washington, and four grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society.