How is GDP counted between different countries?
So, from the link posted by @Brian Romanchuk, I got this paper and got the answer:

GDP = Consumption + Investment + Government spending + eXports – iMports

So in the example, country A GDP would end up to be:
GDPA_A = 3 000 - 2 000 = 1 000 and being correctly a smaller quantity than country B's GDP.

Thanks to contributors