2005 Grand Prix cannot add anymore coolant?

Does it have heat now and it runs fine? Then your topped off. Today you add coolant from the coolant reservoir. Just draining the radiator requires only a gallon of coolant. You said you flushed the system, how did you remove all the coolant form the block and heater lines and core?

"how did you remove all the coolant form the block and heater lines and core?" That's what I'm wondering, you switched from Dex to another type of coolant, you better have every bit of Dex out of there, it does not mix well with other coolants, and draining coolant by opening radiator petcock or removing lower radiator hose is not going to drain engine or heater core completely.

I drained the radiator, then removed the upper and lower hoses one at a time, flushed forward and backward with a garden hose (until the water was running clear) I then blew out the water that remained using low air pressure, again, both directions. I removed the high and low sides of the heater core lines and I flushed with water and low pressure air in both directions (some good chunks of dex came out).