What assumption$($s$)$ in the production possibilities model may lead the model to overstate the gains from trade and cause the actual gains to be less than predicted by the model?

The assumption that there is only a two good economy. In reality the economy is much more complex, so the gains from trade will in fact be larger than what the model predicts.

The assumption that individuals will consume all that is produced. This leads the model to overstate the gains from trade because in reality items will not be consumed.

The assumption that all resources are converted into the other sector. This implies that there is no idle resources in the economy. Therefore, there is no unemployment in the model and all production facilities are running at $100$ percent capacity. In reality there will be unemployment and idle production facilities because all of the land, labor, and capital can't be converted into the other sector.