His minor league career lasted seven seasons in the Chicago White Sox and Philadelphia Phillies' organizations.

He entered pro baseball as an undrafted free agent and debuted in 1972 with the Appleton Foxes of the Class A Midwest League, posting a 3.27 earned run average in eight appearances and registering one save.

In 1973, Gregson was with Appleton (0–4, one save, 4.67, 16 hits allowed, 33 bases on balls in 27 innings pitched) and the Pulaski Phillies (1–4, 4.62).

However, Russell was fired on June 22 and three days later, Gregson was replaced by Charlie Hough on new skipper Glenn Hoffman's staff.

[1] Gregson returned to the Phillies as roving minor league pitching coordinator (1999–2000) and coach for the Triple-A Scranton/Wilkes-Barre Red Barons (2001).