No, a gearbox oil cooler is not part of X51 as far as I know. Even the X50 doesn't have a gearbox oil cooler, even if some people (even owners) still believe that. The GT2 gearbox and 996 Turbo X50 gearbox are completely DIFFERENT.
If you want to retrofit X51 to your 996 (MY 02 and up), this might be difficult (expensive !) to do. I don't have all details yet, unfortunately.