The inter-dependency between the gas and electric grid is growing as the reliance on the gas-fired generators increases. Strong coupling between two large infrastructures requires a coordinated planning and operational framework. In fact, the constraints of both grids must be respected, while their objective functions are co-optimized. In this paper, we present a new formulation for Gas and Electric Grid Coordinated Unit Commitment (GECUC). We apply the standard DC power flow approximation and propose a new formulation to relax the nonlinear gas grid constraints so that the GECUC can be mapped into a Mixed-Integer Linear Program (MILP). In addition, the N -1 generator contingency security constraints are extended to include the gas grid constraints and find the required reserve from gas wells and generators after the loss of a generating unit.