DH and I have gold Cartier watches with substantial bracelets (his is yellow gold and mine is rose gold). We bought them about 15-17 years ago. They look the same as the day we bought them, but we do not wear them in the shower or ocean and we occasionally give them a buffing with a polishing cloth.
Gold is an alloy and the metals that comprise the alloy can and do oxidize. Yellow gold, for example, gets darker/more yellow as it oxidizes, but a quick buffing brings it back to its original color.
I own a lot of 18K gold jewelry and if it is left in the safe long enough without wearing or buffing it, oxidation can form, particularly near clasps and hinges, where a slightly different alloy may have been used to attach parts. Buff that area with a quality polishing cloth and it should clean right up.