To track your cash purchases you can create an "Asset" account, within Account > New Account.
When you withdraw cash, you can enter this as a transfer from the relevant account (for example your Checking account) into the 'Cash' Asset account.
To create a transfer, enter the Asset account into the "Category" field of the transaction. Transfers between accounts are detailed in this article.
This will deduct the amount from your account (e.g. Checking) to represent the withdrawal, and increase the amount in the 'cash' Asset account.
As you spend cash, enter regular expense transactions in the Asset account to represent this.
This means that the cash you have withdrawn still counts towards your net worth until you spend it.