Gain = amplification.
Acquire means to attain - or, yes, gain. But it isn’t meant that way in this context.
From Wiki:
“Gain-of-function research (GoF research or GoFR) is medical research that genetically alters an organism in a way that may enhance the biological functions of gene products.”
Genetic enhancement in that sense, sounds like ‘componentially’ changing the way an organism works, Changing an organism in that fashion, would essentially make them a different organism. Enhancement means making a species’ components to work better, but with the same components in place, sort of like a human does by doing body-building.
Yet, the Covid virus, or the corona virus, is still essentially the same virus, and the gain of function virus in question, is still the same virus with enhanced features that it came with. Bigger, better, faster, does not mean a different organism has been created. Covid-19 had/has variations since it came to infect people in 2019, but, those variations did not result in a different species. Does gain of function result in a different species, or are we talking about ‘gain a function’, which would mean just an enhanced virus with the same components?