gain: profit or advantage, the return a person counts as an asset on the credit side of the ledger
This is a merchant's word. It names what a person comes out ahead by: profit, money earned, an advantage set down on the credit side of an account. In everyday Greek it carried a double edge. On one side sat honest earnings; on the other, greedy or crooked gain, the profit chased at any cost, close to the grasping the New Testament names with a different word (πλεονεξία).
The New Testament keeps both edges and then does something bolder: it turns the ledger over. The word belongs to a pair, gain on one side and loss on the other. Ordinary reckoning files a person's money, standing, and achievements under gain. Paul reverses the columns. The credentials he once counted to his own credit he now enters as loss, because next to knowing Christ they are worth nothing, and the one profit he still wants is Christ himself, whom he elsewhere speaks of longing to gain (κερδαίνω). Pushed to its limit this makes even death a gain, the moment that true profit is finally in hand.
So the same plain word runs from the shameful money of a false teacher to the deepest good a person can be given. What changes is not the meaning of gain but the account it is measured against.