No, GB-PVR doesnt use its own codec. Infact GB-PVR doesn't do any playback or decoding itself. Instead it just asks directshow to do it for it, which in turn uses filters that are installed on you machine. You could emulate what GB-PVR does by starting GraphEdit, and selecting "render file" from the "file" menu, then choose your MP3 file.
I've never installed an MP3 codec, and have just used the default one available in XP.