The TRAPPIST-1 system host seven terrestrial exoplanets in a very compact configuration,where all the planets are very close together, giving a strong gravitational perturbation between the planets. This leads to a large transit timing variation (TTV) signal which can be measured by telescopes in space or on ground. To process the TTV signal into the determination of the planetary masses, involves to process a very large parameter space with a Markov chain Monte Carlo method, and requires a large computational effort. We use the GPU Nbody integrator GENGA, together with a parallel Differential Evolution Markov Chain Monte Carlo (DEMCMC) method to perform this analysis. With this approach, we are able to determine the masses of the TRAPPIST-1 planets within 5% - 12% (Grimm et al. 2018).