Wow - they sure rounded that number up, didn't they? It would have been much more accurate to state that, "...just over 500,000 GTOs were sold."
Here is breakdown by year (all body styles):
1964 - 32,450
1965 - 75,352
1966 - 96,946
1967 - 81,722
1968 - 87,684
1969 - 72,287
1970 - 40,149
1971 - 10,532
1972 - 5,811 (including the 3 wagons)
1973 - 4,806
1974 - 7,058
Total - 514,797