I think that it's important to distinguish between the two different kinds of reporting involved here. The FINCEN reporting is required when the total value of all monetary instruments being brought in or out of the U.S. exceeds $10,000. But Customs requires that one declare all gold being imported into the U.S., regardless of its value. From the link I posted above:

"There is no duty on gold coins, medals or bullion but these items must be declared to a Customs and Border Protection (CBP) Officer. Please note a FINCEN 105 form must be completed at the time of entry for monetary instruments over $10,000. This includes currency, ie. gold coins, valued over $10,000."