Gold never drops or rises as much as the GDX.. the reason for this is margins. Gold miners rely on margins to make their dow.
e.g. if miner A has an all in sustaining cost of $1000, that fall from $1760 to $1200 is actually a fall of $760 margin to $200 margin (~76% fall). Given most miners are above $1k AISC and that some value is given to stocks for assets, potential and commodity futures, this mis-alignment is always going to happen.
also if gold went to 2400 (30% rise) the margin of the gold miner would double (100% increase)