Bubblegum in Mummulgum

This is a 2x Stage Multi Geocache. The first waypoint is a fixed sign.
Reading the sign, it has 4x numbers on it. (A,B,C,D)
Work out the following equation for GZ
GZ -
S28 51.D,A,(A+A)
E 152 47.(B-A),(C-A),(CxC-A)
NO room for trades/swaps or Trackables. BYO Pen.