Ok simply put:
The replay file is like those in Counter-strike
It only tells what and where the car should go/do.
It's made of commands and macro's, not a video.
Replays in games are never video files, they are a set of recordings of inputs and coordinates that the game uses to essentially re-run the race without you playing, but using what you did to control the car as AI commands. Therefore, the only way to view a replay is only ever in GT5P.