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

After trade, the world price P_W is lower than P_d (but still higher than where DD=MC). Thus the good is infinitely imported or exported with price P_W.

The domestic monopolist produces where P_W=MC as P_W=MR: in other words, it produces Q₂ and consumers consume Q₃. Then the amount exported is Q₂ - Q₃. (2)

As a result:

Consumer surplus changes from AP_dB to AP_WE.

Producer surplus changes from CP_dBH to CP_WF.

The net gain is BGE+HGF (3)