We have studied a big karstic cave (Grotta Gigante) in northern Italy using an innovative combination of laser-scan and gravity data. We aimed to forward model the gravity anomaly due to the cavity, verify its compatibility with the Bouguer field, and identify the eventual presence of other sources of gravity anomalies. A sensitivity study was performed preliminarily to assess the minimum size of bodies that could be detected by the gravity surveys. The 3D density model of the Grotta Gigante was constructed using as a geometric constraint the laser-scan data set, which mapped the internal morphologies of the cave, and density measurements on collected rock samples. The laser point cloud was reduced in data density, filtered from the outliers, and subdivided into two surfaces representing the vault and the floor of the cave, to correctly define the prism model. Then, a mean density value, obtained from laboratory measurements, was assigned to the prisms. We computed the gravity effect of the model in the same points at which the gravity field had been measured. Excellent correlation was found for the cavity; some gravity anomalies were revealed in the surrounding area of the Grotta Gigante that could be effected by other underground karstic morphologies. We attempted to estimate the probable size and depth of the causative bodies, compatible with the geologic environment. This site testified to the goodness of gravity methods for the exploration of such structures, that is, particularly important for risk assessment in a karstic area. The cave itself, the biggest tourist cave worldwide, represents an upper limit for expected gravity signals. The combination of exact knowledge of the causative body and the related gravity anomalies composed a unique data set (that we released to the public, as a benchmark), useful for testing inversion and forward model gravity algorithms.