Is your coolant level full in both the rad. and the overflow? My GT did this when the rad. was leaking and the level would drop only when I was on it or cruising above 65mph. As soon as I let off and coasted for a second the light went out. Back on the gas=light again.
Replaced the rad and bled the system 3 times before I could get all the air out of it. If your coolant is full then I would try bleeding it again.
And no, headgaskets are not a common issue on this motor. The LIM is far more the issue, then coolant elbows. Also check your waterpump for any leaks. Might be hidden and getting spun off by the belt/pulleys.
