Gabe was the 271st employee at Microsoft[0] and "was the producer on the first three releases of Windows"[1], so I suppose that, even if he hasn't contributed to the very same code as Bill Gates did, there must be an earlier employee who has. I don't estimate his Gates code number to be more than 2.

[0] http://www.giantbomb.com/gabe-newell/3040-4498/

[1] http://www.computerandvideogames.com/172835/interviews/creat...