Before trade, this country produces where MR=MC, with production Q₁ and price P_d. (1)

After trade, as the domestic price P_d is greater than the world price P_W, domestic consumers consume Q₃.

As P_W is the amount of money that producers receive whenever they sell one more unit, the domestic monopolist will produce when P_W=MC: in other words, Q₂. Then the amount imported is Q₃ - Q₁. (2)

As a result:

Consumer surplus increases from AP_dB to AP_WE.

Producer surplus decreases from CP_dBH to CP_WF.

The net gain is BGE+HGF (3)