What happens if I use the Huge Rock to pay the Income Tax?
An item is any treasure card that has a GP value or no value in the bottom right corner.  In this case, yes, the equivalent to "No Value" is 0 gold.  
You must discard a card to satisfy Income Tax.  Since the Huge Rock has no value, you must discard at least one card with a value of at least 0, so basically any item will work (note that cards like Impressive Title don't even state "No Value", so it's not an item).