It is not the price of gold. What you are seeing is the last traded price of that SGB in the secondary market. This depends on the supply demand of (different) SGBs on secondary market - irrespective of the underlying price - you can actually take advantage of this.
Price of SGB is equal to price of Gold only at the time of issue and at the time of redemption. Redemption is different from selling it in secondary market. Redemption of SGB can happen only after 5 years of issue, the process of which I’m not actually aware