what are 'gil's-eye bandits'?

In Season 1 episode 4, Brydon was talking about visiting the villages in the mountains near the Afghan border and says satirically "the hills are alive with gil's-eye bandits." I know that the reference is to the Sound of Music song "the hills are alive with the sounds of music..." but someone tell me what "gil's-eyes" are? I tried googling it but didn't come up with any results. Thanks!