gold Cd's are remastered by audiophile companies, which can correct flaws in the original record. That usually explains why they sound better than normal alluminum cds.
As for the gold itself, i'm pretty sure it has very little effect on sound. Some people that paint their cd's green and freeze theirt cd's think the reflectivity of the material is very important and determines how well a cd sounds. I think that's BS, so i think the gold itself does not improve the sound. However, the discs are manufactured on better equipment, hopefully resulting in less jitter/bit errors, but i doubt that there is enough of a diference for an audiable difference.
When CDs first came out, there were problems with aluminum oxidation, and CDs could degrade within a few years. Gold is much more resistant to corrosion than aluminum, and should last longer. However, most manufacturing problems with aluminum cd's have been solved, and i think they will last around 50 years...