I have all three gold cd's.
To the one who asked how all they differ from the originals, they are remastered from the original master tapes by an audiophile label called Mobile Fidelity Sound Labs. Their objective was to make the album sound more faithful to the master tape. Believe it or not, the sound quality you get on CD is a much more compressed and lesser quality version of the master tape. MFSL tried their best to make it sound better.
As for the Unforgettable Fire gold cd, it isn't any better than the original cd to be honest with you. Perhaps the master tapes were bad to begin with, or the mastering done by the U2 techs in the original cd was as good as it could get already. So in short, you're not missing anything.
For the War cd, you get a few bonus seconds in the song "Seconds" (or was it Surrender?), I'm not sure. And the sound quality is an improvement over the original non-gold CD.
However, it is really with the gold Joshua Tree cd where the sound really shines. It is a totally new listening experience all together, the drums sound more natural, there is more air around the instruments, and the bass is not muddy or boomy. The vocals are more solid and less grainy compared to the original.
To discern these differences you need to have a decent playback rig. Playing it in your car stereo, or computer speakers, or discman headphones, or your boombox, or anything Bose won't make you notice these minute yet vital nuances and improvements. I highly recommend the Johsua Tree gold remaster but not the Unforgettable Fire. You aren't missing anything with the Unforgettable Fire gold cd.
Cheers,
J