Geomechanical simulations for the management of underground resources involve multiple physical processes, such as fluid flow, poromechanics, fault activation, thermal flow, and chemical reactions, that can take place simultaneously with multiple time and space scales. GReS is a novel open-source modular platform, specifically designed with the aim at contributing to the development and prototyping of numerical algorithms for fully coupled multi-physics multi-domain geomechanical applications. The idea is to partition the overall computational domain into possibly non-conforming subdomains where different physics and discretization schemes can be used. The code is based on a high-level programming platform (MATLAB) that should lower the entry barrier for new users and developers, as well as the effort for implementing and testing innovative numerical algorithms. Moreover, the modular structure of the code encourages contributions from different developers at variable levels, from the implementation of new physics and discretization schemes to specific algorithms to accelerate the linear and non-linear solver. Innovative implementation of the mortar algoriothm are used to glue together non-conforming subdomains and to simulate the non-linear contact behavior between non-conforming fracture surfaces. Despite being primarily conceived as a prototyping platform, GReS wraps low-level advanced linear algebra packages to combine simplicity with fair efficiency. In the present communication, we will introduce the GReS concept and its current development state, with particular reference to the advances on the mortar algorithm used to transfer the information among non-conforming surfaces with independent meshes. A few benchmarks will be presented to show the current code’s potentials, along with the projects for future developments.