supply of gold is not affected for its industrial utility and its "capped" due to industry grow and miners. bitcoin is not instant, it take times to verify the transaction in the mining side, often days for low transactions.
Discussion
it is capped because tomorrow you can not open a gold mine and increasy the gold supply in such a way to disrupt the market, first of all because you don't have the machines and mining heavy equipment and second you need humans to do that job.
It is capped because gold it is not used in industry and it is not expected we are going to use our gold supply to cover industrial needs in short or long term.