It is a wonderful idea to use the fine meshes locally in a global coarse mesh domain, which is called subgridding technique. However, many efforts in the recent years on this topic have not applied into practical applications due to suffering the later time instability. Using an advanced transferring technique from the coarse to fine region, GEMS provides a stable subgridding module together with implicit technique, which avoids the reduction of the FDTD time step. Since the implicit technique is adapted in the update procedure, it will be more time consuming inside the subgridding than outside. Therefore, we always use as small as possible subgridding region to achieve a better code performance. In this demo, we guide the user to use subgridding technique in GEMS simulator step by step.