shades of grey

Well it certainly is an eye catching scheme. And since it is from a video game, you dont have to "match" any 'official' colors per se.  So I would suggest go with what looks right. For the darkest gray, something along the lines of Exta Dark Sea Gray, gray RLM 66, or FS # 36081. For the Medium Gray, something akin to Dark Gull Gray FS# 36231, RLM 75, or some other medium neutral gray shade. for the lightest shade I would suggest Light Gray "FS# 36495.

Like stik suggests, it's all pretty much down to perception.

What you could also do is compare the colour from the image to the greys on the FS Colorserver, get some numbers & use the something like the "Ultimate Model Paint Conversion Chart" to get matches to your preferred paint brand;

http://www.colorserver.net/

http://www.paint4models.com/