The Honeywell SS49E looks like it is quite linear from -1000 to +1000 Gauss. The analog input range is 1/5 scale to 4/5 scale so 205 to 819. You can translate to Gauss with:
int Gauss = map(analogRead(HallPin, 205, 819, -1000, 1000);
Note that outside the 205 to 819 range it is likely to be non-linear so values under -1000 or over +1000 should not be used.