I believe gain corrections are saved in a tag that just says the up/down dB level. And as such the other info should not be affected.
Then there is the issue of compression. If you take two songs with relatively big compression diffences, that have the same true gain level, they can (and usually will) sound vastly different. So, what you PERCEIVE to be the loudest track might actually have the same gain level. I am not sure that autogain functions (all) compensate for that.
So far I have been using Platinum Notes (see article on blog) and it does a very nice job. Not just autogaining, but also correcting clipped signals, key corrections and since version 4.x adding “warmth” as an option. Not cheap, but high quality.
Sure Terry can help you out with Serato specifics, it’s his thing :-).
Greetinx,
C.