Something is "funny" on the checkout_payment when the customer has a GV balance and the store uses authorizenet_aim (and possibly others; I'm guessing that it's any payment that accepts card data on-site) as a payment method.

Choose a customer who has a GC balance, so the GC block is displayed on the checkout_payment page. Go through the checkout steps to checkout_payment, don't select a payment method and click the "continue" button. The form submits without displaying the javascript alert that indicates that no payment method was chosen. The page is subsequently re-displayed due to the checkout_confirmation page's error-checking.

Under the same conditions, if the customer doesn't have a GC balance, the javascript alert is properly displayed.

The issue appears to be that the submitter variable is being set due to the simple presence of those GV-related values.