GM-GMC: 2006 GMC Sierra coolant smell, overheats, and heater not working., coolant level, hose clamps

Question
I have the 6.0L with auto tranny, and about 60k miles.  Recently I noticed a strong coolant smell, but I can find no leaks.  This morning I could hear a fan running while driving on the freeway.  This evening it started to overheat on my way home.  Minimally on the freeway, but quite quickly on the slower side streets.  The heater stopped blowing hot air.  When I stopped at home I could see steam coming from the front right side of the engine, but I was unable to pinpoint the exact location. ( i was too short and it is dark out)  What does this sound like to you. Please tell me this is a simple thermostat and not some crazy heater repair.  Thank you for your time.

Answer
There are a number of possibilities, the heater is not likely one of them.  The reason you lost the heat in the cab was because the coolant level dropped to a level below the bypass hoses for the heater.  When the coolant level drops, it directs all the coolant to the import place (the engine).  As is said there are a few places that they tend to leak.  Top up the cooland and let the truck warm up or untill you can feel some pressure in the rad hose.   Turn the truck off and have a look to see where the steam you said was coming from.  Check the water pump, and the water pump gaskets, the front of the cylinder head, and check all the hoses and clamps that enter or exit the engine.  I would say the most common problem is broken hose clamps, the hose starts to leak and you loose coolant.  The least likely and most costly would be a headgasket, but I have seen very very few.  Check those three main areas.  The water pump, the water pump gaskets and your hoses and clamps.
Hope this helps