Please explain?
A zero sum game is a game in which one player's gains have to be balanced by other players' losses.
As an example, think of the Six Nations Championship. There is a total of 30 points available which means an average of five points per team. However, England ended up with 10 points this year, which means that the average of the other teams points was reduced to four.
Global economics is not like that. If Romania's GDP goes up, it doesn't mean some other country's GDP has to go down. In fact, when a country gets richer, it generates demand for imports of foreign goods which means that other countries also benefit.
So, if a company making widgets moves production from Britain to (say) Spain transferring 10,000 jobs to Spain, initially, there are 10,000 more unemployed people in Britain and 10,000 more employed people in Spain.
In Britain, the extra unemployed puts downward pressure on wages and the exchange rate making British companies more competitive abroad, improving our economy. Furthermore, some of those 10,000 unemployed people will set up new and perhaps innovative businesses and many of the rest will find jobs elsewhere. Perhaps some of them even move to the Spanish operation.
In Spain, where unemployment is still at around 21%, 10,000 people who were living off the state are now in gainful employment with disposable incomes. This provides a boost to the Spanish economy and people start buying more stuff, including from Britain. Furthermore, widgets are now being made more cheaply, so that anybody who uses them gets a benefit.
If you add up the net loss to Britain and the net gain to Spain, you will find that the combination of the two countries is better off than it was before.