Commonly, we get calls from customer that are an issue that is totally unrelated to the Cadillac CUE system or the touch screen.
This Red slash GPS issue is one of those. It may have coincideded with the screen replacement, but it is unrelated and replacing the touch screen again will not resolve the issue, it will simple waste a lot of time.
If there is no GPS, then your antenna on the top of the vehicle is not communicating. Sometimes the seal on the antenna leaks water in the connection and does not allow position lock. I would start looking at this connection, and replace the cable or the antenna if you see any signs of corrosion.