Complex sentence: The gallery was established in 1961, by Jeremy Rees (1937--2003), son of the artist Jean Rees and moved to its current site in 1975 and has established itself as leading centre for contemporary arts.
Split into simpler sentences: The gallery was established in 1961, by Jeremy Rees (1937--2003), son of the artist Jean Rees and moved to its current site in 1975. It has established itself as leading centre for contemporary arts.