It is good that you ask this question before coming. Some information you did not state is whether you are just a tourist, or importing for a business.
Gold buillon does not fit into this category.
The South Korean Government takes a very hard view on people bringing Gold buillon into the country, whether declared or smuggled.
If you are a tourist, chances are against you; South Korea confiscates all Gold buillon; you'll never see it again.
Just last week on a Korean TV program they showed the Immigration Officials at Incheon Airport dealing with smugglers. Items found were uncut and cut diamonds not mounted into settings. Other items included fake designer-name items.
And yes, they also found gram-sized bars of Gold buillon amongst the items being smuggled in.
To be safe, contact a South Korean Government or Immigration internet site to ask this question.
You'll definitely need Government paperwork to bring Gold bullion into the country.
Get the paperwork you need to avoid confiscation forever.