Gartrell was born near Washington, Georgia. He attended Randolph-Macon College, and Franklin College (now known as the Franklin College of Arts and Sciences), the founding school of the University of Georgia in Athens. Gartrell passed the state bar in 1842 and began the practice of law in Washington. Gartrell served as the solicitor general of the northern judicial circuit from 1843 until 1847 when he was elected to the Georgia House of Representatives. He was subsequently elected to the first of two consecutive terms in U.S. House of Representatives in 1856.