Hitchcock was born in Omaha, Nebraska. He attended the public schools of Omaha and the gymnasium at Baden-Baden, Germany. He graduated from the law department of the University of Michigan at Ann Arbor in 1881; he was then admitted to the bar and commenced practice in Omaha in 1882. He continued the practice of law until 1885, when he established and edited the Omaha Evening World; four years later, he purchased the Nebraska Morning Herald and consolidated the two into the morning and evening editions of the Omaha World-Herald. Hitchcock was an unsuccessful Democratic candidate for the U.S. House of Representatives in 1898; four years later, he was elected as a Democrat to the Fifty-eighth Congress (March 4, 1903-March 3, 1905). He was an unsuccessful candidate for reelection in 1904 to the Fifty-ninth Congress. Hitchcock was elected as a Democrat to the Sixtieth and Sixty-first Congresses (March 4, 1907-March 3, 1911).

He did not seek renomination in 1910, having become a candidate for the United States Senate. Hitchcock was elected as a Democrat to the United States Senate January 18, 1911; he was reelected in 1916 and served from March 4, 1911, to March 3, 1923. During his two terms, he was the chairman of the Committee on the Philippines (Sixty-third through Sixty-fifth Congresses); he also served on the Committee on Foreign Relations (Sixty-fifth Congress) and the Committee on Forest Reservations and Game Protection (Sixty-sixth Congress). Hitchcock was unsuccessful candidate for reelection in 1922 and for election in 1930. After the end of his Senate service, he resumed newspaper work in Omaha. He retired from active business in 1933 and moved to Washington, D.C., where he died on February 3, 1934. He was interred in Forest Lawn Cemetery in Omaha.

Gilbert is the son of Phineas Warren Hitchcock, who also served as a U.S. Senator from Nebraska.