The first Global Navigation Satellite System Occultation Sounder (GNOS) which is compatible of both BeiDou System (BDS) and Global Positioning System (GPS) was successfully launched into orbit onboard the FengYun 3 C satellite (FY-3C) on September 23, 2013, and it has already gathered a large amount of ionosphere radio occultation (RO) data so far. However, the detailed analysis and validation of GPS ionosphere RO data have not been done up to now. Therefore, this paper discusses the configuration of the FY-3C GNOS, the methods and results of GPS ionosphere occultation processment, the quality analysis of the GPS ionosphere RO products, and the precision consistency between the GNOS GPS ionosphere RO product and ionosonde data. The peak electron density (NmF2) correlation coefficient is 0.97, the corresponding standard deviation is 16.08%. The peak value altitude (hmF2) correlation coefficient is 0.89, and the corresponding standard deviation is 23.79 km. The in-orbit operation of GNOS provides a basis data set for the monitoring, forecasting and research of ionosphere’s space weather. © 2018 Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature