We present and apply a trans-dimensional inversion method for 3D gravity inversion. This method builds on a multiple level set inversion approach using signed-distance functions to model the location of interfaces between rock units. To account for the unknown number of rock units that need to be modeled, we use a death and birth process where rock units can be inserted into or removed from an existing geological model. In addition, we also invert for the geometry of rock units and their densities. We test our method using a synthetic model before applying it to field data from the prospective Boulia region (Queensland, Australia) to image rock formations under sedimentary cover. In this field application, we start from an implicit geological model derived from the interpretation of 2D seismic lines, borehole data, and geological rules that presents a relatively strong gravity data misfit. Preliminary results indicate that up to 2 rock units that were not initially identified may need to be added to the model. We argue that this shows the ability of our method to infer the presence of unseen geological features such as inclusions or facies variations.