Question example in BPP P2 text book regarding IAS 19 Employee Benefits

In page 140 of BPP P2 text book for exams in 2012, there is a question about defined benefit plan (DBP). There is a gain on settlement in this DBP: “Assets with a fair value of $48,000 were transferred to the other company’s plan and the actuary has calculated that the reduction in defined benefit liability is $50,000”. In the solution, they recognize this gain in SOCI as an addition to the expense (i.e decreasing P/L). Im wondering whether this is an error, if not, why a gain becomes an expense in this case.

Thank you very much!