Gilby made his television debut in 2016, when he played a village lad in an episode of Jericho. In 2017, he starred in the film Just Charlie, playing Charlie Lyndsay, a transgender girl with a special talent for playing football. He was nominated for Most Promising Newcomer at the British Independent Film Awards 2017 and Best Newcomer at the National Film Awards UK 2018 for his performance in Just Charlie. In 2018, he played Arlo in the short film Evie. In 2019, he played a young version of author J. R. R. Tolkien in Tolkien. He has also appeared in three episodes of Casualty as Toby Williams, a young boy dying of leukaemia. In 2020, the role of Robin Martin in an episode of British detective drama Granchester was played by Gilby.
In 2022, he played Æthelstan in the fifth series of The Last Kingdom. In 2023, he reprised the role in the follow-up feature-length sequel Seven Kings Must Die. In the same year, Gilby played the role of Eddie Holdgate in the short film ManMade, for which he won the Best Actor award at The Soho London Independent Film Festival.
In 2024, Gilby played the role of Rupert in BBC Three show and will return in the second season of the show.