Kanryo Higaonna (1853-1915)
Possibly developed the kata after training in China for 13 years and taught it to Miyagi.
Chogun Miyagi (1888-1953)
It was most likely the founder of Goju, Miyagi developed the kata after studying one of the three internal styles of Kung-fu.
Gogen Yamaguchi (1909-1989)
Gogen Yamaguchi was already a reputable karate-ka of Goju before he met its founder (Chogun Miyagi) around 1930. Miyagi became so impressed with Yamaguchi that he made him his successor for Goju in Japan.
While also all styles hailing from Naha practise Seiunchin, GKR derives its version from Japanese Goju (not the more kung-fu appearing Okinawan Goju Seiunchin).