Actually, yes it is a Faraday cage. The penetration depth is approximately equal to the size of the holes in the grid. That is, it will work next to a window, not elsewhere. But that's a restricted view of the sky, so it won't always work (you'll need at least 4-5 satellites in that direction, which is far from guaranteed)
As was mentioned, the Faraday effect is depended upon for lightning protection.
The aircraft's own GPS antenna (if it's equipped -- not all airliners are) is on top of the roof, and is therefore outside the cage.