Ben (Big Ben) Gillery, a 7-foot-1, 240-pound basketball center at Hutchinson (Kan.) Junior College, has signed a letter of intent to attend Georgetown University, it was learned yesterday. He becomes the first junior college player signed by Coach John Thompson in more than a decade.

Thompson was unavailable for comment, but an assistant coach whose school was among the finalists for Gillery said he thought Gillery would not have an immediate impact on the Hoyas but has the potential to be a "very good player in that system" in two years.

Last season, sharing time at center, Gillery led his team in blocked shots and averaged 8.2 points and 6.0 rebounds per game, shooting 57 percent from the field and 53 percent from the foul line.

Gary Cramton, an assistant at Hutchinson, said Gillery's strength was as a low post player, both offensively and defensively. Cramton also said he was unaware that Gillery had signed. Among other schools trying to recruit Gillery were Indiana and Oral Roberts.

Gillery is one of two members of his family signing letters of intent to play at Washington schools. His younger sister Tina, who is 6 feet 5, recently signed at Howard University.

The last junior college player signed by Thompson was Bill Thomas in 1974.

Meanwhile, George Mason announced the signing of three recruits, including the school's tallest player ever, 6-10 1/2 Henri Abrams from Mackin High School. The Patriots also signed Steve Smith of Kennedy High School, a 6-4 swing man, and 6-2 Brent Carter of Grissom High in Huntsville, Ala.

Abrams, 17, played only his second season of organized basketball this year and averaged six points, nine rebounds and six blocked shots.

Smith averaged 17.7 points and 9.7 rebounds in helping his Montgomery County school to a 17-7 record.

Carter, a third-team all-state player, averaged 22.5 points, five rebounds and four assists and scored 30 or more points four times this season. He set a school and city record when he scored 51 points against Albertville.