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What is the opposite of a sunk cost? A “sunk gain”?
It is called "Opportunity Cost."   Opportunity cost is the value you lose because of a decision you made.   This is the book definition from Investopedia. The difference in return between a chosen investment and one that is necessarily passed up. Say you invest in a stock and it returns a paltry 2% over the year. In placing your money in the stock, you gave up the opportunity of another investment - say, a risk-free government bond yielding 6%. In this situation, your opportunity costs are 4% (6% - 2%).