Well, both Winamp and foobar2000 work for me, and you can get multiformat plugins that play GBS for Winamp at http://www.zophar.net/utilities/audio-multi/nezplug-.html and foobar at
http://www.zophar.net/utilities/audio-multi/game-emu-player.html. I read that the foobar one was more accurate in emulation, but it kind of hurts my ears. The Winamp one sounds less harsh to me and I compared both plugins to the OST release of the SaGa soundtracks and Winamp's is closer, but you can make your own decision.
Sorry if any of this seems obvious and I'm treating you like you're stupid. Trying to make it clear and complete.
In Winamp, go into Options, Preferences, and Output then select the diskwriter. I don't think you'll have to change it, but make sure the format is set to WAV. It is possible to convert to MP3 with Winamp, but the encoder that comes with the free version only allows awful quality and I had no luck with the third-party MP3 writer plugin. So I got LameDropXPd (
http://www.rarewares.org/mp3-lamedrop.php) to convert them, which is a cool program. The tagging component is kinda stupid though.
With foobar, you have to select the files in the playlist and right click to select convert. It'll ask you where your MP3 encoder is located. Click the "..." to change the options and select your format. Go to RareWares (
http://www.rarewares.org/mp3-lame-bundle.php) and download one of the LAME MP3 bundles and then point foobar to the .exe file. You can change the encoding quality level in the options with the Edit button. Regrettably not as customizable as LameDrop though.
Both players can mass tag files. In Winamp you have to import them to your media library and select multiple ones, right click, yadda yadda. In foobar, simply select, right click, yadda yadda.
You should be good with either of those ways. Report back how it goes. You can ask me about any other option or whatnot and I'll answer as best I can.