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Perlman has no known history as a comic book writer, but she worked for Marvel Studios from 2009-2011, and various sources indicate that she was the one who first saw the potential in Guardians. Her treatment impressed Marvel Studios enough that they commissioned a full screenplay which she wrote and Gunn later modified. Perlman didn’t work directly with Gunn--it was more a case that he made enough changes to her script that they both received writing credits for the final film.
Jim Starlin created Gamora way back in 1975 in Strange Tales #180, where she first appeared as one of Thanos’ minions (after all she was his step-daughter), though she later turned against him, joining with the Avengers to take on the Mad Titan.