Decommissioning is the last step in the life cycle of a nuclear facility. After the evacuation of the facility components, the remaining structures such as concrete walls and floors must be surveyed to ensure that no residual contamination remains. It is a costly and time consuming activity, for which CEA develops fast alpha and beta detection methods allowing a full scanning of very large areas (hundreds of thousands of square meters) in legacy uranium enrichment plants. To support these developments, we present here complementary high-resolution gamma-ray spectroscopy analyses of a contaminated area at the gaseous diffusion uranium enrichment facility UDG, currently under decommissioning at Pierrelatte nuclear plant, France. Long measurements are performed with a High-Purity Germanium (HPGe) detector on the contaminated surface, and in a clean area to assess the natural gamma background of the concrete ground. The surface activity of uranium is 16.6 ± 6.0 Bq.cm-2, mainly due to 234U and 238U, most of the uncertainty coming from the non-uniform distribution of the contamination on the ground. This measurements also allowed us estimating the uranium enrichment of the contamination, which amounts to (0.80 ± 0.13) % of 235U mass fraction, consistently with the range of the Low Enrichment Plant where this measure was performed. Eventually, the background spectrum allowed us to determine the mass fractions of natural uranium, thorium and potassium in the concrete ground, which respectively amount to 3.8 ± 0.2. ppmU (i.e. 3.8 mg of uranium per kg of concrete), 7.4 ± 0.7 ppmTh, and (2.6 ± 0.1) %K of potassium.