In the past, networks of natural gas transmission system operators (TSOs) determined the gas market areas in the European Union. However, gas markets mergers introduce the possibility to book the transmission capacity of alternative TSOs. One necessary condition for competition among TSOs is the absence of restrictions in capacity booking. This paper analyses whether this holds for Germany. As German TSOs distinguish a number of capacity types to deal with network constraints, market mergers have created transport alternatives for only 32% of cross-border capacity products. In almost all cases, we find that gas transmission network users make efficient booking decisions.