While I can't speak for the GAC on this, as a GAC member, and as a former student of the topic, I can provide my understanding based on my role in my country's Government if that might be helpful.
In my country, and in many parts of the world, the term, Public Policy is not used. In many countries (as in mine) Public Policy = National Policy.
Public or National Policy IS NOT EQUIVALENT to National Law, however, it is the key and normally most critical precursor to the development of legislation in the Government law making process. If we were to stretch the concept somewhat, we can say that National Law (when promulgated) forms a key element - but still a subset - of Public or National Policy.
In summary and simply, Public or National Policy is a Government's statement of direction - usually with clearly stated objectives and deliverables - on a particular area or series of areas - that is intended to be reflective of that country's (citizens') norms, attitudes, cultures and values; and further, to be implemented (and regulated and enforced, where applicable) by a country's various institutions primarily consisting of the Executive, the Legislature and the Judiciary. As a rule of thumb, National Policies should be developed through extensive consultation with a country's stakeholders over a significant period of time and several iterations of National Policy are to be expected before becoming "final" (and potentially leading to legislation). In other cases, National Policy is time limited and subject to periodic cycles of consultative redrafting and review - with potential attendant changes to related laws and regulations.
Public or National Policy is intended to be reflective of, and protective of, what has often been termed as the "Public Interest".