Gilgun was born in Chorley, Lancashire, to Judith and Andrew. He has two younger sisters, Jennie and Rosie. He attended Rivington VA Primary School and Southlands High School. He also trained at the Laine Johnson Theatre School and the Oldham Theatre Workshop. He has dyslexia and ADHD, which he describes as the "biggest pain of [his] life" and in interviews has openly discussed depression and anxiety. He started drama workshops at the age of eight, following advice from an educational psychologist, and was described as having "exceptional talent". When he was 10, he got his first TV acting role in Coronation Street. He stayed until he was 13. Gilgun studied art at Runshaw College. He started a degree in fashion design, but found it too academic. He was a plasterer, until returning to acting full-time with Emmerdale.