The best way to track your Petty Cash expenses is to set up a new BANK account. With the Chart of Accounts window open, press the Account button at the bottom left (or CTRL + N) > New, and Choose Bank as the Account Type. Write in "Petty Cash" for the Account Name. This will create a register just like your checking accounts.
When you write a check to Petty Cash from your regular checking account, classify it with this new account. This will put money INTO the Petty Cash Account. Then you can enter all of your petty cash expenditures in this register, just like your check register, which will provide a complete record of each expense. At the end of the month, reconcile this register against the amount left in the cash box. (With the Petty Cash register open, select Banking > Reconcile.)