You have half answered your question.
If the oxide (or other corrosion product) stays on the object there is a gain; if it is removed there is a loss, because part of that oxide is metal.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
In some cases there is only weight loss, if the corrosion products are soluble.
This is often true of Cu corrosion.
Also in some cases of very localized corrosion (pitting) the weight change may be very small and hard to detect by weight.