the actuarial gain/loss is the difference between your actual return on plan assets and your expected return on plan assets.

your actual return on plan assets is part of the pension expense calculation.

fmv plan assets beginning of period + contributions - benefits

100 + 30-10= 120

compare 120 to fmv plan assets at end of period of 140

120< 140 = 20 return on plan assets, and reduce pension expense by the difference

if the fmv plan assets were less then the 120 you would add the difference to pension expense.

NOW, this gain you are asking about.

the gain/loss is the difference between your actual return on plan assets(just computed above as 20) and the expected return on plan assets.

however if you remember to always use the lower of actual return or expected return, then there is no need to apply a gain or loss to your pension expense calculation since that gain or loss is just the difference between the two components.