The GR6 is a strong transmission, however it DOES have its problems. We make a living out of selling repair parts and designing fixes for them, so have a fair idea of what you can expect if and when something goes wrong. The OP in this thread is hardly Robinson Crusoe when it comes to having issues. Its not one in a million.
Common problems (when they occur) are -
Inadequate lubrication causing valve body contamination and subsequent P0731-P0732 limp mode
Piston seal failures, P0700 onwards codes and limp mode
Four gear synchromesh failure - the scrapnel causes missed shifting on fourth gear and P0731 and upwards codes
Line Pressure/Clutch Pressure sensor failure - P0830 (off the top of my head)
FWD Circlip Failure - catastrophic transmission failure or if you are lucky just a damaged main shaft
FWD transfer shaft Failure - P0700 and onwards.
Oil Pump Failure - Harsh grinding noise
Transmission case failure - as the name describes
We have repaired 2 x GTR transmissions locally in the past fortnight with a selection of these issues, and obviously shipped parts from Dubai through to Hungary (and most everywhere in between) to fix over a dozen in the same period.
The good news is that most of these problems can be avoided with preventative fixes