I took a quick look at gnucash and couldn't find anything on payroll. Accountant's copy was not found either.
As for the restaurant itself, you may have a touch time finding Point of Sale (POS) software.
Quickbooks is good, but you may also want to consider Simply Accounting. For some, the multiple currency is great (extra for Quickbooks). Some differences in pricing.
I would be careful of online software. Gets expensive paying every month and you are at the mercy of the provider.
Backup your data.
As some have suggested, use software on the market and donate to open source.