AFAIU (prolly not much), GDP measures money that comes and goes to/from a country. If, let's say, products/services were exchanged directly, without moving money, that wouldn't be counted; OTOH, if certain country, say the US, gave money to another country only to buy products from US companies, that would count doubly... Which is why it's really bad as a tool for comparing wealth of nations.
Please correct me if I'm wrong.