Common misconception to be honest.
Polyphony is using (was using?) what was then one of the two The Illuminate Labs plug-in products - either Turtle or Beast or both, we don't know that - basically a middleware lightning plug-in for global illumination effects for PD's own proprietary lightning engine and help with some Maya rendering.
In the meantime (2010) The Illuminate Labs have been bought by Autodesk Company and now that particular solutions have been integrated into what is today an Autodesk Beast global illumination (GI) middleware.
Last time any mention of GT series using that solution was during Prologue development phase where Prologue screenshots were even used for print-advertising of The Illuminate Labs in specialized print.
However, since Autodesk bought The Illuminate Labs, Gran Turismo is nowhere to be seen on their official web nor it lists their solutions on the credits.
Worth noticing is how Polyphony did not "use" some european company for their environmental lightning but they've used their software in order to enhance their own engine. The Illuminate Labs solutions were backthen used also by EA, Blizzard, Epic, SQ and many more while today Autodesk Beast is one of the most common middleware solution in this gen.
Some examples from Autodesk's developer section using Autodesk solutions (
size alert):
http://area.autodesk.com/userdata/image/showcase/h/7g9cb-73k85-k68e3-gsy33.jpg
http://area.autodesk.com/userdata/image/showcase/a/qpk27-5b2ec-ukar6-7z2d2.jpg
http://area.autodesk.com/userdata/image/showcase/p/r5g88-f2576-7aqvr-48tc6.jpg
Guy named Luis Nieves did this beautiful video of GT40 using Autodesk Maya plugin:
http://www.youtube.com/watch?v=Ktayh4vpC_U