A combined gas and electricity network expansion planning model was developed. Gas-fired generation plants were considered as linkages between the two networks. The model simultaneously minimises gas and electricity operational and network expansion costs. Additionally it optimally places planned power generation plants around the electricity network. Network expansion was implemented by adding new assets such as pipes, compressors, and storage facilities in the gas network and increasing transmission line capacity in the electricity network. The developed model was used to analyse the GB gas and electricity infrastructure expansion requirements to achieve a low carbon energy system. Two scenarios were implemented, a reference and a low carbon scenario. For both scenarios, CGEN defined a network at lowest cost capable of meeting varying demand and power generation capacity profiles. Greater peak gas demand of approximately 25 mcm/d by 2030 in the reference scenario resulted in CGEN adding an additional 1 Bcm of gas storage capacity compared with the low carbon scenario. LNG gas supplies were shown to account for over 70% of total gas supplies by the end of the time horizon for both scenarios. The combined gas and electricity network planning approach allows analysis into the interactions between these two networks. This interaction allows variables such as total gas supply (gas used for electricity production is an endogenous variable), which 13% higher in the reference scenario and geographic location of gas fired generation to explicitly take account of the impact on both gas and electricity infrastructures.