I'd like to add replay gain tags to my entire flac library. What software can I use (Windows GUI or command-line) to do this? It will need to traverse the entire directory structure and add both track and album gain tags. Albums are organized one per directory.
foobar (http://www.foobar2000.org/) would most likely be the easiest way..
You can use the metaflac command line utility to do it.
metaflac --add-replay-gain *.flac
Would work fine in any *nix type system, but the metaflac binary itself doesn't expand wildcards (instead relying on the shell to do that), so using Windows you would have to do something like:
metaflac --add-replay-gain Track01.flac Track02.flac ... Track10.flac
Not quite the easiest way to do it, but it could be possible to automate the process with scripting.
I would go with Cyaneyes' suggestion - Foobar would be your best bet.