Well, technically, gain is a unitless item of measure. It’s Watts/Watts or BTUs/BTUs or whatever/whatever, with the units cancelling. That’s first year chemistry & physics.
But it really would be meaningless if the units were small enough, say microwatts/microwatts. Because the level of noise at that realm is significant. So, yeah, it is sloppy, but it’s also not likely to be microwatts/microwatts, either.
A microwatt here, a microwatt there, pretty soon your talking milliwattts.........