Create an Investment account by selecting Account->New Account.
Call the account "Precious Metal".
Set the initial balance on the account to the initial purchase price of the metal.
Create a security to represent 1oz of silver. Do this by selecting Tools->Edit Securities.
Set the security name to "Silver".
Set the ticker symbol to "SLV".
Set the value suffix to "oz".
Click on the investment account.
You will be asked if you want to add a security to this account. Click Yes.
Select the security you just created. Set the Type to other and create a new sub type called silver.
Click on the "Register" view.
Click New transaction and enter a buy transaction for the number of ounces of silver you own at the price you paid for them.
In the screenshot I purchased 300oz at 20USD/oz
Add the Stock quotes extension by selecting Extensions->Manage. Scroll down until you see the "Quotes and Exchange Rates Updater" and click "Install"
Select Extension->Quotes and Exchange rates.
Change the update frequency to "Yearly". This will download the last year of prices.
Click Update Now.
Change the update frequency to back to "Daily" to update the value of your precious metal(s) each day.
You can download example files below.