A disruption mitigation system is mandatory in ITER in order to reduce electromagnetic forces on the structures, mitigate heat loads and avoid the generation of runaway electrons which occurs during disruptions. Massive Gas Injection (MGI) is foreseen in ITER to trigger a "controlled" disruption in order to greatly reduce the potential damage on the tokamak compared to an unmitigated one. However, mitigating all effects of a disruption at the same time is challenging. Modeling is therefore needed to provide insights into the physics involved and inputs to support the design of the MGI system in ITER. This article is divided as follows: the first part presents simulations of JET disruptions triggered by MGI with the 3D non-linear MHD code JOREK. The second part is devoted to simulations of the penetration of the neutral gas into the plasma with the 1D fluid code IMAGINE.