GDP is not itself a measure of welfare, though in some circumstances changes in GDP correlate fairly well with changes in measures of welfare (see for example Coyle D GDP: A Brief but Affectionate History pp 113-122). So it's a fallacy to argue that if the increases in exports and imports do not change GDP they cannot be good or bad.
If, for example, the initial position was one of autarky, and if the exports are of a good in which the country has a comparative advantage and the imports are of a good in which it has a comparative disadvantage, then welfare can be raised (as indicated for example by higher consumption from the same domestic inputs).
As a secondary point, especially for a small country which must accept world prices for goods it exports or imports, trade will result in changes in at least some domestic prices. That in turn means that it may not even be true that GDP would not change, since some of the prices used in the value added calculations to measure GDP (from the output perspective) will have changed.