Certificate of Origin or Form A is an evidence of a country of manufacture. GSP stands for the Generalized System of Preferences. The US Congress set up the GSP classification in the early 1970s. It was designed to help scores of developing nations to speed up their economic development by allowing their exports to enter the United Sates duty free. The only requirement was that a particular country be first approved for the GSP list of countries and that the appropriate Certificate of Origin, Form A, be attached with the shipment. To meet such a standard was not only based on the economic criteria but also subject to meeting certain political guidelines. That meant that even though a particular country could meet the economic criteria of being a developing nation, having a communist government in power or being subject to social form of instability such as civil war, be prone to violence and human rights abuses, being governed by an oppressive military regime or similar manifestations of unfavorable political connotations, the country was either not included on the GSP list or thrown out of the GSP list.
If a country is on the GSP list, you, as an importer, can import goods from that country duty free provided that your shipment is accompanied with the Form A document. If, however, your shipment arrives without such a document or the country loses the GSP classification while your goods are in transit, you will have to pay duty on your goods based on a classification out of the Harmonized Tariff Schedule under which your goods are entered through the US Customs. The only products, typically, that do not qualify for the duty free exemption under GSP are garments and textile articles in general. The only nations that are excluded from this limitation are the those that are classified as as a Least Developed Beneficiary Developing Country under the GSP.