The ongoing worldwide outbreak of high pathogenicity avian influenza (HPAI) H5 of the Goose/Guangdong (Gs/GD) lineage has caused unprecedented mortality amongst an ever-expanding number of avian and mammalian species. This includes black-headed gulls, which suffered high mortality from this virus in 2022 and 2023. We performed pathological and virological analyses on black-headed gulls and a common tern found dead in a breeding site in The Netherlands in spring 2023 in order to characterise the pathogenesis of HPAI Gs/GD virus infection in these species. We found a strong tropism of HPAI H5 Gs/GD virus for the central nervous system, pancreas, lungs, and the preen gland, with acute necrosis as the main associated lesion. Affected organs revealed high virus RNA loads and intralesional expression of virus antigen, confirming HPAI H5 Gs/GD virus infection as the cause of disease and mortality. Genetic sequencing showed that this HPAI H5 Gs/GD clade 2.3.4.4b virus belonged to the gull-adapted BB genotype. Comprehensive pathogenesis studies in a cohort of wild animals naturally infected with HPAI H5 Gs/GD virus are scarce, yet the knowledge gained may fill knowledge gaps to help mitigate future HPAI outbreaks.