"Gains trading" involves

moving securities whose value has decreased since acquisition from available$-$for$-$sale to held$-$to$-$maturity to avoid reporting losses.

reporting investment securities at fair value but liabilities at amortized cost.

selling securities whose value has increased since acquisition $($winners$)$ while holding those whose value has decreased since acquisition $($losers$).$

all of the above are considered methods of "gains trading" or "cherry$-$picking."