A human being can gain a function that he/she did not have before. Body building can make a person a lot stronger than before. That’s gaining a function. A virus that is enhanced is gaining a function, with the same properties it already possesses. So, why should it be called ‘gain OF function’ when it’s just enhancing a property that already exists in the organism?
No, body building involves enhancing a function that is already there - the ‘stuff’ required to do it is there; so it’s a gain (or increase) OF function.
I’m tired of this now. Find someone else to convince that the term is a misnomer.