2000 Pontiac Grand Am Chronic overheating

I bought this car from the original owner with 212 K miles. Only known problem at purchase was slight coolant leak. This has become a problem of overheating frequently, requiring a jug of Dex-Cool in the trunk always. Leaks can't be seen, but you can smell anti-freeze. Mechanic changed water pump and thermostat to no avail. Interior heat is not very good on highest setting. When driving you can smell anti-freeze, but if you put the air selector on recirculate (MAX) the smell is gone, so it must be coming from under the hood and sucked in by the blower. Just today I spoke to a friend who said his daughter had the same car and Pontiac has an un-advertised warranty to change the head gaskets to fix this problem, which they did on her car at no charge. He said mileage is irrelevant for this warranty.