The presence of high penetration of Variable Renewable Energy Sources (VRES) is one of the key aspects of the modern electricity system. The new challenges to be faced require novel technologies which enhance the flexibility of the transmission system. In this paper, the exploitation of the power-togas technology (PtG) is considered as a solution for the flexibility challenges, allowing to absorb the excess of electricity produced by VRES and at the same time, producing synthetic natural gas (SNG). This work presents a computational framework based on DC Optimal Power Flow capable to simulate the day-ahead market and the following intra-day market, applied to a simplified European transmission network, by considering different scenarios for both load and generation. PtG plants model are modelled based on the real measurements of a 2-MW Alkaline (AEC) electrolyser. The results of a given PtG placement configuration, applied to a current scenario (2017) and two future scenarios (2030&2040) show that the fast response of PtG units improves the system performance and reduces the VRESs' imbalance up to ~90% in terms of imbalance time duration and ~40% in terms of peak imbalance power. These results lead to further investigations, about the optimal PtG units' placement and sizing, as well as their economic and technical consequences on the transmission network operation.