they show the spot price and USD rate because that's the currency gold is traded in on the international market. The Thai price is directly related to the USD exchange rate of the moment.
You should also know that when buying and selling there is fee, typically 100 baht per baht weight on bars, some shops may lower if you buy bulk, some shops may be higher, ask their fee first. Same goes for jewelry but the fees are higher depending on the shop and craftsmanship of the jewelry, typically 1500-1800 baht per baht weight. It goes without saying that jewellery is not a good investment buy.
This is the website that sets gold prices in Thailand. Prices are set daily at approximately 9am and follow the market fluctuations. Price is set only once Saturday morning and that price is maintained until Monday morning.