I can't say for sure but I suspect that the only difference is a camshaft change. The G version likely has a little stronger torque rating at a lower rpm. Considering the G wieghs so much more I think I would leave it that way or you are likely to lose performance.
The 3.6 is based on the M103/104 engines and the bellhousing will be the same. That would be a great upgrade. I would confirm which 3.6 version you are considering to swap in. On a 300GE I would want the single cam M103 based version just ofr ease of conversion. The twin cam M104 version has more power but will also require the electronic control units to make it run. That would be a slightly more involved change.
The 3.6 is based on the M103/104 engines and the bellhousing will be the same. That would be a great upgrade. I would confirm which 3.6 version you are considering to swap in. On a 300GE I would want the single cam M103 based version just ofr ease of conversion. The twin cam M104 version has more power but will also require the electronic control units to make it run. That would be a slightly more involved change.