GDOP (geometric dilution of precision) or PDOP (position dilution of precision) describes the error caused by the relative position of the GPS satellites. In simple terms, the value of GDOP defines the accuracy of GPS position.
If the satellites are CLOSER you will have a HIGHER GDOP and LESS ACCURATE POSITION
if the satellites are FAR APART you will have LOWER GDOP and MORE ACCURATE POSITION

HDOP (Horizontal Dilution of Precision) is a part of GDOP and concerns with the horizontal distance between the satellites