The honorary co-captain of the Purdue Boilermakers in both his junior and senior years, Gilliam twice received Purdue's MVP award and was the recipient of the Ward Lambert Scholarship Trophy for scholastic excellence. In three seasons at Purdue, the 6'3" 190 lb guard scored 1,118 points and finished his collegiate career in 5th place among the school's all-time scoring leaders.
Gilliam was selected by the Chicago Bulls in the 13th round (3rd pick) of the 1968 NBA Draft, and by the Cincinnati Royals in the 1st round (8th pick) of the 1969 NBA Draft. He won a championship ring with the Portland Trail Blazers during his final season in 1977. After retiring from the NBA, he took a job with United Parcel Service where he worked for 15 years, eventually being promoted to managing the company's Corvallis-Albany hub.