That is only if it buys bread and milk. The trouble Greece and most of the modern world has is that its entirely dependent on international trade. Greece can't meet its needs by itself. I am not an expert on the Greek economy. Lets charitably assume they can feed themselves. What about all the drugs that are not manufactured there that many depend upon to live for example? Can a private individual order drugs from across the boarder with gold coins? Can a pharmacy or hospital buying in quantity for that matter?
Sure there are exchanges for gold abroad, ultimately the answer is yes; for some quantity of gold you can obtain enough Euro to buy what you need. Now if the banks are closed where you are that might mean sending someone abroad to physically execute these transactions where trading desks and banks are open.
If the economy becomes truly unhinged, people stop working, stores close, etc than gold really is not all that great. If I am hungry and you are hungry, and neither of us imagines that changing anytime soon do you think I'll trade my pound of cheese for your gold?
I am supportive of a gold standard in general because I think inflation and debt based currency is an insidious trap used to enslave all of us. A gold standard would prevent the vipers from manipulating things and causing recessions that last half of peoples productive lives, it would reduce inequality, it would reduce war, in exchange for more frequent smaller booms and busts. In short it would shrink many of the worlds problems. If you already have problems like Greece does it won't provide some magic fix, don't have any illusions about that.