This is a weird thing you're describing.
I use the manual mode on my 05 GT once in a while. From what I understand you can shift into this mode at any time and at any speed as it is not a truly "manual" mode, it is instead a self selected automatic mode. I don't know if you noticed but when you press on the brakes and come to a stop in manual mode it automatically downshifts to 1.
Having owned both manual and automatic cars I prefer a true manual with a clutch, the system on the G6 is slow and clunky. The only thing it is really good for is engine braking when going down very long hills, or holding gear through a turn.
I'm pretty sure that the shifting part is electronically actuated either at the base of the shifter in the car or on top of the transmission where the shift cables connect. You could take a look and see if there is anything stuck there like a pinecone or something, otherwise its probably an electrical fault.
Aftermarket Exhaust system w/chrome tip. (came with car)
Red ZZ Performance Strut Tower Bar.
Pioneer DEH-P5200HD CD Receiver (HD Radio).
Metra 99-3303 adapter and Steering wheel control adapter.
LED bulbs in climate control panel.
Moda MD9 wheels, Falken Ziex ZE 912 tires.