Ok, so you were in 4th gear, at what speed? How long did you stay on it? 4th gear in these high hp cars is moving pretty good, and unless you shut it down at 150+ at that point, it wasn't really a good race. Not to mention, he was behind you and probably didn't get to react to your taking off for probably 2 seconds. That means you already had him by 30mph, which means he had to also make up for that as well.
I attempted to race a Mustang GT in a similar circumstance and actually had trouble because of these issues. I know for a damn fact my car will toast a GT too. By the time I made up for the difference and was about to reel him in, he was shutting it down.
Lastly, are you sure he was racing you?
Just some thoughts.
He drives a newer GT500. There equally matched stock for stock. A GT would not even be fair if not boosted. Lol. I'd like to run a GT500 in my hellcat but mine is somewhat modified