I should also point out that no one has been working on or updating MP3Gain since the beginning of 2005. As such, at some point, the various changes it makes to the MP3 files and additional tags may now longer work correctly or may cause other problems with playing while playing the files. The reason MP3Gain may (eventually) no longer be of any use is because of the on-going changes to the ID3 tag specifications.
As more and more players start using the newer ID3 tag specs, there is a chance that they may no longer be fully backwards compatible with the way MP3Gain was designed to do things (way back in 2005 with an older and now outdated ID3 tag spec).
On the other hand though, the people behind Replay Gain are still continuing to work and update their stuff to keep up with the ID3 tag spec changes. So, Replay Gain stands a much better chance of remaining funtional in the future.
Things are now changing far to rapidly with all forms of technology these days. So, what worked yesterday may no longer work tomorrow. It’s getting really hard to keep up with everything and it’s difficult to keep everthing updated to the newest, latest, greatest, whatever.
