The phrase 'zero-sum gain' is like fingernails on a chalk board for me. I just read it on a worksheet handout for an upcoming RWA workshop on Professional jealousy. It is a common error of the English language and shows that the person using the phrase doesn't know what it means and is simply throwing the words about -- a malapropism.
The actual phrase is zero-sum game. Specifically, the phrase describes a type of game in game theory. I had to do game theory when I did my econ degree so indulge me here.
In a zero sum game -- if one person wins, the other loses. For example tic-tac-toe is zero sum. A cake is zero sum. If one person has a large piece, everyone else remaining must share less. The outcome is finite. Nothing is created, nothing is destroyed. One person wins and the other loses.
Romance writing is not like that. Romance writing is not a zero sum game because one person's success does not mean that everyone else loses. Things are created. The genre is infinite. There are always new publishing slots.
Romance is more like -- a rising tide floats all boats. It is a non zero sum game. Non zero sum games are actually far more interesting.
In order for one person to be successful, it doesn't mean that everyone will be less successful. The outcome is dependant on many factors.
It is why romance authors can take joy in others' success. They are not losing anything. It is not a one shot deal.
BUT please, please, please use the correct phrase and know where it came from.