During the filming of the dinner scene towards the end of 'Texas Chainsaw Massacre', Gunnar's character Leatherface was supposed to cut the finger of Marilyn Burns' character with a knife.
Gunnar was having trouble working the fake-blood dispenser and was very fatigued by the heat and long hours of doing take after take, so unbeknown to everyone but him and Marilyn, he simply cut her for real, which is the take used in the film.
Was asked to reprise the role of Leatherface in The Texas Chainsaw Massacre (2003), but declined, claiming that he was insulted by the idea of the original 1974 film being remade.
Worked as a computer operator outside of high school
After acting in the movie Demon Lover, he decided to quit acting and continue his dream of writing, but after being offered many jobs that he refused (including Hills have Eyes and Eaten Alive) he decided that he could both be an actor and a writer.
Said in a TV interview that he is so unlike his TCM character that when people who meet him learn that he played Leatherface, they often react in total shock.
Was a football player in high school; also worked as a bar-bouncer prior to acting.