Though the F40 6-speed manual was used in quite a few cars models, the numbers were small, many years have passed and Pontiac parts are unobtanium. The most common request is for shifter cables, which run from the shifter handle assembly to the transmission.
I found this Fiero Performance parts company: Fieroguru Performance LLC | Specializing in Brake and Suspension Upgrades for Pontiac Fiero
Apparently, the F40 has been used as a stronger replacement transmission for Fiero's and they make a shifter kit that allows it's use with the Fiero shifter (handle). From the pictures, it looks like it includes the cables. Might be worth contacting them to see if the cables will or could work on a G6 with the F40. Good Luck.