What plays USB without gaps between tracks

The issue is entirely with the device you're using to play it (in your case, that specific Bluray player). Definitely check first if the player has the option for "gapless playback" or something to that effect. Another kludgey thing you could try is burning the music files to a CD with gapless playback enabled, because sometimes players will handle CD format gaplessly but not music files. If you're willing to go the laptop/phone route, it's much easier to find a program or app with gapless playback

Thanks