If an exchange is internal to the unit under analysis it is counted as GDP. If the same exchange happens between two units it is added to the GDP of the exported and subtracted to the GDP of the importer, so on a net basis it is zero.

For instance, Scotland selling oil products to England counts as UK internal trade and so increases the UK's GDP. However, if the UK broke up, Scotland selling oil products to England would add to the GDP of Scotland and subtract from that of England.

Therefore, Scotland selling oil to England contributes to world GDP through the UK's GDP, but Scotland selling oil to Ireland doesn't have a net effect on world GDP. Unless, of course, world GDP is the sum of national GDPs plus the value of international trade. Is that how world GDP is calculated?

We have met the enemy, and he is us — Pogo