The simplest way I can (try to) understand it is to imagine a line drawn between CERN and Gran Sasso. From the GPS point of view(consider the GPS satellites as

The simplest way I can (try to) understand it is to imagine a line drawn between CERN and Gran Sasso. From the GPS point of view(consider the GPS satellites as stationary), this line is moving approximately in a Gran Sasso->CERN direction, and is therefore very slightly shorter than what the researchers on the ground calculated