Update: SCEE's just offered VG247 this response.
"We haven't confirmed this and aren't offering any other comment at this time."
Original Story: According to E4G, Polyphony has confirmed that a track editor will be available in Gran Turismo 5. At least, thats what the developer could have said to the site.
Anyways, you will be able to either fully edit from scratch, or just edit the tracks already there, apparently.
"Yes it will have a track editor. You will be able to rebuild a track from scratch or edit the tracks that are already put in Gran Turismo 5," said a rep from PD to the site.
The force is telling us something is wrong with this, hence why this is labeled as a rumour, so we've emailed SCEE to confirm or deny this.