Gaddy was a young man playing with two younger men, Clark and Barker, in veteran Jarman's quartet. They first played together in public in 1966 and were together for only a few times in the course of, I'm sure, less than a year - Gaddy's death was the group's end. He was so distinctive, such a colorful pianist, as in Jarman's "Non-Cognitive Aspects" (one of the very best jazz-poetry recordings, BTW) - yet not just ornamental or "meditative". His colors fit and enhanced Jarman's alto-sax virtuosity, his active lines and harmonies had their own weight. Definitely original. How many other pianists back then excaped the influences of Cecil Taylor or Bill Evans? I believe Larry is right about the debilitating condition - Gaddy did not have self-destructive habits. IIRC Clark was 21 when Song For was recorded and Barker was younger.
Charles Clark was a kind of outside Mingus with a big tone, force - presence, as King Ubu would say. He was a favorite of Jarman and Abrams and played often with Braxton. He also was the bassist in the first Roscoe Mitchell concert I ever heard (also 1966). A Braxton group with Leroy Jenkins, Clark, Barker, and usually Maurice McIntyre played often in clubs and concerts. Clark did less jazz work after he began studying with Guastafeste and playing in the Civic Orchestra. Different stories I heard back then about Clark's death included stroke and heart attack. The Chicago Symphony then established the Charles Clark scholarship for talented young African-American musicians to study with CSO members.