The Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) is an international reference observing network of sites measuring essential climate variables above Earth's surface, designed to fill an important gap in the current global observing system. GRUAN is envisaged as a global network of eventually 30-40 sites that, to the extent possible, builds on existing observational networks and capabilities. Currently there are 26 GRUAN sites available. GRUAN is truly a global collaboration among many institutions around the globe to host and operate instruments and sites, and make contributions to GRUAN’s success through involvement in GRUAN working group and task teams. Ground-based GNSS-PW was identified as a Priority 1 measurement for GRUAN. The GNSS-PW Task Team (TT) was established in summer 2010 as one of the six GRUAN TTs. Its goal is to develop, update and assist implementing explicit guidance on hardware, software and data management practices to obtain GNSS-PW measurements of consistent quality at all GRUAN sites. GFZ was identified as GRUAN central processing center for GNSS PW in 2013 and is responsible for processing GRUAN GNSS raw data and producing GRUAN GNSS PW product. GRUAN data product (GDP) is characterized by the following four unique requirements. The raw data and measurement metadata are retained in a secure archive such that they can be reprocessed at any time in the future to generate a revised final data product. All aspects of the measurement processing are open and transparent. The measurements are traceable to SI units or accepted community standards. More importantly, the uncertainty in every processing step is identified, assessed and robustly quantified to provide a comprehensive estimate of the uncertainty on each measurement. The GNSS PW uncertainty estimate was developed and summarized in Ning et al (2016), and will be implemented in GFZ data processing. This talk will give an overview of GRUAN GNSS efforts and focus on GNSS PW GDP including data flow, centralized data processing, and PW uncertainty estimate, validation and applications.