He studied at the Karolinska Institute, becoming a doctor of medicine in 1953, and was an associate professor of histology there from 1955 to 1973.

His first written work, published under the pseudonym Jan Wictor in 1946, was a collection of poetry by Gyllensten and Torgny Greitz entitled Camera Obscura, a straight-faced parody of Swedish modernist 1940s poetry.

The Swedish Academy biography refers to his "dialectic" prose trilogy Moderna myter ('Modern myths', 1949), Det blå skeppet ('The blue ship', 1950) and Barnabok ('Child book', 1952) as the "real" beginning of his authorship.

Gyllensten left the Swedish Academy in 1989 as a result of its failure to support Salman Rushdie following the fatwa calling for Rushdie's death because of his controversial novel The Satanic Verses.

According to the rules of the Academy, Gyllensten remained a passive member for the remainder of his life.