what are gold cd's

A gimmick, mostly. It's the same old Redbook. No they are not comparable to HDCD, unless they happen to be HDCD. And oxidation is a nonissue.

Edesilva got it right--High-end producers use gold to signify their better products. But the disks aren't better because they're "gold." They're better because they're well-made/mastered.

Gold is infinitely malliable so it has nice plating or foiling ability. Also it should have a longer shelf life. But I got no objective data.

IMO: The gold was a marketing gimmick. Most gold CDs were released by MFSL and DCC. In many cases they do sound better than their aluminum cousins but not because of the gold. They were generally remastered from original tapes by some great mastering artists such as Steve Hoffman, Bernie Grundman etc. Many consumers would have a problem paying twice the price for these remastered CDs so the gold was touted as an extra benefit. JVC has proven that this was un-neccessary and that premium remastered CDs do not have to be gold to sound their best. I doubt that the gold alone contributes much to the sound quality of a CD. In fact, some gold CDs sound terrible.

I've seen the same titles in Gold and Aluminium from MA Recordings. I'm doubt they differ in mastering. In price - yes. Any other difference I'm missing?

possible that gold metal has better optical reflectivity (than aluminum) for infrared laser, just a guess...