Gastronorm is a modular sizing system for food-service containers and compatible equipment. The full GN 1/1 footprint is 530 by 325 mm; smaller fractions and larger sizes fit the same system. Depth is specified separately.
A GN designation describes dimensions, not a guarantee that every material can be heated or frozen. Confirm the pan's temperature rating and equipment fit. Similar-looking US hotel pans may have different dimensions.
Reference: Further reading.