The use of Global Navigation Satellite Systems (GNSS) will provide significant advantages for ensuring the control of train movements on railway networks equipped with ERTMS/ETCS. Indeed, this disruptive technology can play an important role for reducing the rail infrastructure's equipment by enabling trains to determine their position in an autonomous way. Specific associated hazards have to be considered in the European signaling system. From this perspective, performances of GNSS-based localization systems were analyzed in different past studies. They highlighted what they can bring to the railway domain. For going further than performance-centric analyses to help inserting these systems safely in rail applications, this work want also to focus on their operating uses. For that, the safety evaluation proposed in this paper is able to handle specific estimated confidence-related data that are associated to an estimated position. This article firstly details the managed risks in ETCS due to the localization function with, in particular, those that may arise due to GNSS-based systems. It discusses the evolution of these risks when dealing with moving block operation of the ETCS-level 3. The safety evaluation approach is then explained. It relies in fact on extended integrity data handled in the case of the train spacing. Finally, it is applied in different operational cases to show evaluation results when real GNSS data are measured in a railway environment.