An equity gain is a positive variation in your assets. It is a key figure in income tax returns when it comes to paying taxes on investments.
So much so that there is a specific area of savings income in the personal income tax (IRPF in Spanish) that is dedicated to equity gains and losses. From a tax point of view, equity gain occurs when there is a change in wealth in favor of the taxpayer.
An example of a equity gain would be the sale of shares in which the transfer price exceeds the purchase price. The same applies to the sale of a house.
When filing your income tax return, you will have to pay tax on the profits or capital gains you have made.