Just read your post Nick. Yes, GM used dexcool in your car and it is crap. It is known to cause problems with the lower intake gasket. What I would recommend based on what you wrote is to completely flush the system of dexcool. It's an easy process. Drain all the dexcool from the system and then run clear water from a garden hose through the system. Be sure to run the car so it gets hot and turn the heat all the way up but have the fan off. This will ensure getting dexcool out of the heater core. Once you see no more dexcool coming out then shut the car off and let it cool down. Reconnect all hoses and fill it back up with 50/50 mix of regular antifreeze. Keep in mind your Malibu has a bleeder valve so once it is hot you'll need to bleed the air out.
You wrote about gunk being in the antifreeze. My concern here would be a bad lower intake gasket. Which means your oil and antfreeze are mixing together. Take a look at the inside of your oil cap and the overflow cap. If there is a mustard looking goo on them then your intake gaskets are leaking.