There is no kit to convert to a manual transmission on the G6.
The short list is that you would need the transmission (duh) plus the clutch, clutch master cylinder, hydraulic lines between them, the clutch pedal interlock switch (keeps you from cranking the engine unless the clutch is disengaged) a smaller brake pedal, a new insert for your console, shift knob and linkage, and possibly at least part of a salvaged wire harness as the newer models may not have the connectors needed to hook up the manual transmission.
I'd be more concerned about whether the car would run after you swapped the trans. Your car has the transmission control module integrated with the ECU. You might have to find an ECU from a manual trans-equipped G6. The ECU gets reprogrammed no matter what as gear ratios change (speed sensor) and the ECU has to be told it is coupled to a manual. And of course there is hooking up all the interlocks, gear sensors, etc.
It would be a pretty expensive conversion, maybe around 2 - 3,000 by the time you were done.
~ MattInSoCal