Gene Ontology Consortium data and data products are licensed under the Creative Commons Attribution 4.0 Unported License. A human-readable version and explanation is available at the Creative Commons website. For information about how to properly credit data use, please review the Creative Commons FAQ or contact the GO Helpdesk.
That said, the GOC wishes the users and consumers of GO data publicly display the date(s) and/or version number(s) of the relevant GO files, data, or software version. The GO is evolving and changes regularly--this information is critical to downstream consumers and users.
GOC software and tools are under their own licenses; please see their respective homepages for further details.