Gains differ from revenues because why?

Very simple.

You buy an item for $20.

You now sell it for $30.

Your revenue is $30 (what you got from selling it).

Your gain is only $10 (You got 30, cost you 20)

Gains differ from revenues because why?
revenues are income from the normal operating cycle of the entity.

gains on the other hand are income that comes from extraordinaary transactions.

for example, if the normal operating cycle of the business is based on merchandising. Its revenues would be derived from the sale of goods in the inventory. On the other hand if they sold their service vehicle above its book value the sales price above the book value would be considred as a gain.