Gains, sometimes called gross profit, is what the winning positions produced before the losing ones take any back. If 40 positions closed in the green for a combined 4,200, gains is 4,200, full stop. Whether the losers gave most of it back is a separate column.
Reporting gains and losses separately matters because the shape of each side is its own story. Two strategies can end the year on the same total profit and arrive there in completely different ways. One might have produced a few large winners that carried a fleet of small losers. Another might have piled up many small winners that kept ahead of a handful of painful losers. The total at the bottom is the same. The position-by-position behavior is not.
Gains is also the numerator of the profit factor. Divide gains by gross losses and you get a single number that tells you which side weighed more in absolute terms.
Like every historical metric on the platform, gains describes what already happened, on a real account, on a specific period. It is a measurement. It is not a forecast.