07 G35 6MT can't shift into any gear with engine running but except engine off

[G35MoonRiver]

115422 miles. Has been Sitting in my garage for 5 weeks. Clutch seems engaged ok. As I shift into gear before fire up the engine it moves no problem, but can't even push the stick into any gear with engine idling. Engine runs ok. It had to be forced to shift particularly in low gears until the transmission warms up.
Any ideas?

[G35MoonRiver]

Fixed.
It's the starter interlock switch didn't have enough clearance to the clutch pedal, resulting in the pedal stopped by the switch stem, it couldn't go further down, couldn't disengage clutch completely.
How did it come to this? Back in about a year ago the plastic disc on the pedal shattered, so the switch couldn't be pressed down to turn on the starter. To fix it I replaced the shattered disc with a metal bolt, BTW, I did the same to another shattered plastic disc for cruise control cancel switch (ask me how long those cheap plastic discs last).
But this time the head of the bolt was too thick to have enough clearance to the switch stem, so it stopped by the switch too early before the clutch fully separating the flywheel clutch disc, before cutting off the input power to transmission completely. So it ended up that when the engine running, its power to the transmission couldn't be cutoff to let the synchronizers work.

[AZhitman]

I'd bleed the clutch first (and change your clutch fluid).

Next, I'd drain and fill the gearbox with the correct fluid (no telling what's in there).