2003 Silverado Coolant Loss

My 2003 Silverado has 128k on it, since about 80k it started showing the low coolant warning and I have added as I go since I was not seeing any leaks. I am running the prestone green. I recently replaced the intake manifold gaskets, about two months ago and seemed to have fixed it, which is even wierder because I could not find any coolant passages in the plastic creation. the only coolant ports that i could see were on each corner of the block with the two blocks in front having a metal transfer tube in between heads. Iassume this and the many portals on the water pump are how this engine, (325) is shifting coolant. i am obviously or so i think, burning the coolant as my truck seems to have that condensation exhaust, like cold winters, even on the warmest day. yet it takes two weeks to get through the coolant and have to add to it. also i can here the heater core gurgling, i recently checked the plugs which should have some green tint or look like they were pressure washed if i had head gasket problems but no evidence at all. Any suggestions before I buy the head gaskets and ONE TIME use bolts? Thanks In advance

I am running the prestone green.

Shouldn't you be using a Dex-cool coolant? I am not sure you should be using Prestone Green it causes corrosion and build up. I know it says it is safe for MOST makes and models.. I suggest flush the cooling system out and put in a Dexacool coolant.

You can look at this link as well.
http://www.popularmechanics.com/how_to_central/automotive/1272436.html

In short it states:
The bad news is that it doesn't come down to color matching. Isn't all antifreeze compatible? Sorry, but it isn't. There are a few cases where you can get a fast case of corrosion, some where the water pump can be both eroded and corroded to an early death, and a lot of cases where you can shorten the effective life of the antifreeze. Antifreeze itself (most commonly a chemical called ethylene glycol) doesn't wear out–ever–but the key ingredients are the rust and corrosion inhibitors, and they get used up. The most common inh ibitors, silicates for aluminum protection, are used up faster than others. That's why some new cars (all General Motors, Mercury Cougar, VW/Audi models) have red or orange antifreeze with a new class of inhibitors, called organic acids. Examples of U.S. brands with these organic acids: Havoline Dex-Cool and Prestone Extended Life, both recommended for five years or 150,000 miles. That's the good news.