GNOSIS is an OH suppression unit that was designed to retrofit the IRIS2 spectrographs. The OH suppression is achieved using fibre Bragg gratings (FBGs), with an aim to remove bright sky lines before they scatter within the spectrograph and therefore deliver the darkest near-infrared background of any ground-based instrument. Laboratory and on-sky tests demonstrated that FBGs can suppress OH lines by 30dB whilst maintaining >90% throughput between the lines, resulting in a ≈4 mag decrease in the background.