The car was overheating. Was the coolant low? Did the shop use a hydrocarbon sniffer in the radiator or coolant tank to determine that a head gasket had failed? Not impossible, but head gaskets hardly ever fail on the 3800s.
Good that you got the Chilton'*. Now read Techinfo 38 and also read the links on UIM/LIM gasket replacement at the end of the Techinfo 38 article linked for you above. Remove the UIM and examine the EGR bore area for softness and warpage on the sealing surfaces. Remove the LIM and change out the gaskets for the GM aluminum ones. There are many, many articles here on BC that you can find using the search feature that will let you know what to expect. Changing out the UIM, LIM gaskets, and coolant elbow will cost you about $200. If the car still has a coolant leak and overheats, and you have to go back in to change the head gaskets, you will be able to re-use all the new parts you installed on the top end. On the very small chance that you will need to go back in, go sparingly on the rtv in the corners only of the LIM gaskets and side rail pieces. That way, you can remove the LIM from the block and heads more easily without damaging them.