When can two countries gain from trading two goods?

The scenarios for the two countries to gain are

1. One country can produce only one good and the other good should be produced by second country.

2. when country produce both goods but the production of the second good is expensive in the country and in the same way second country produce both goods but the production of the fist good is expensive.

3. When just one country is great at producing the both goods and other country is bad at producing both goods