The gross domestic product (GDP) is one of the primary indicators used to gauge the health of a country's economy. It represents the total dollar value of all goods and services produced over a specific time period; you can think of it as the size of the economy. - from investopedia

Unless there is a law that prevents us from using bitcoin ,then It should be considered as legal already and for the moment I don't see how Its helping to increase GDP of a coutry.