Not sure if you guys are interested in such a code for cwcheat.
If you activate the code before the race starts, it'll end once you cross the Start/Finish line.
If you activate after you've started the race and have cross the start line already.. it'll finish when you complete the lap.

_S UCUS-98632
_G Gran Turismo [US]
_C1 99 Lap Modifier
_L 0x21517830 0x00000063
_L 0x21517834 0x00000063
_L 0x21517838 0x00000063
_L 0x2151A114 0x00000063
_L 0x21530894 0x00000063
_L 0x215309E0 0x00000063