With the increasing share of variable renewable generation, balancing electric powersystems could become a major concern for system operators because of their variableand hardly predictable nature. However, gas technologies appear as a solutionto provide this flexibility, but the impacts on the gas power system have hardly beeninvestigated. In this thesis, consulting reports on the subject matter, regulator suggestions andgas-electricity interaction models in scientific literature are studied and four sourcesare identified to be used for balancing: linepack, storage facilities, liquefied natural gasand intraday gas supply from adjacent areas. Then, a gas-electricity model for flexibility supply is designed and three case studies are simulated in order to analyze bothgas and electric power systems’ behaviors. In these case studies, electricity generation,contribution of gas sources and costs are analysed. The study concludes that critical situations on gas market that can occur, e.g. incases of large variation in the net electricity demand and limited availability of linepackand storage facilities, the need of intraday modulation can exceed the possibilities toprovide for it. Then, gas cannot be supplied to power plants during peak periods, andmore gas than necessary is used during off-peak periods. The case studies also showthat day-ahead forecast errors in variable renewable generation can be handled mucheasier than variations by the gas system but leads to higher costs.