At the time of the writing of this guide, this achievement is currently glitched for anyone who has even played the first Rainbow Six Vegas. This is because they get an experience bonus which puts them past the mark for the achievement, making it never unlock. There is currently a way to avoid the glitch if you've never accepted the bonus experience in this thread
If you have accepted the bonus experience you can reset your rank using the method in this youtube video
. I would suggest getting to Elite and getting all other achievements first, then finishing your 1000G with this method and this achievement.