Strategic behavior by gas traders is likely to affect future gas prices. Within this article a computational game theoretic model is presented which allows assessing market power on the natural gas market and its influence on the electricity market. This model uses typical time segments to represent both seasonal load fluctuations on the natural gas market and hourly load fluctuations on the electricity market. An application is presented covering 40 regions and simultaneously optimizing dispatch and utilization of transmission lines on the power market as well as supply, transmission and storage on the natural gas market. The model is used to evaluate the influence of trader market power in the natural gas market on the electricity market. We compute price changes, sales volume and power plant utilization for three different market power specifications.