[5] It is named for Daniel Coit Gilman, the first president of Johns Hopkins University and an early supporter of efforts by Anne Galbraith Carey to form an all-boys day school.

At the Upper School level, students are required to take courses in history, mathematics, English, science, and a foreign language each semester; an intramural or interscholastic sport each season; and a minimum of art and music instruction over four years.

[13] Gilman's Science, technology, engineering, and mathematics (STEM) program was ranked #312 in Newsweek's 2019 nationwide survey of US high schools.

Cross-registration also allows for a variety of languages to be offered, which currently include French, Spanish, Latin, Ancient Greek, Chinese, Russian, and Arabic.

The football team has won 13 Maryland Interscholastic Athletic Association (MIAA) "A" Conference championships in the last 20 seasons.

[16] The 2002 team finished 10–0 and was ranked 14th in the United States by USA Today's Super 25 high school football poll.

[18] That team featured the Associated Press's Offensive and Defensive Player of the Year in quarterback Ambrose Wooden and lineman Victor Abiamiri.

[21] During the 2005–06 school year, six Gilman varsity squads (football, golf, ice hockey, squash, tennis & track and field) won conference titles.

Gilman's varsity athletic teams have won over 120 championships since 1940, including 41 conference titles since the MIAA was formed in 1994.

[17] Gilman confers over 50 awards at the upper school level for achievement in academics, athletics, student leadership, and extracurricular activities.