Traditionally gold has had an inverse relationship to equities as a safe haven against Dollar deflation,inflation and asset decline.
That was, until gold had a fantastic run up after the '08 crash and was the daily talking point of the financial gurus. Then, it hit the ridiculous from sophisticated sounding, well-dressed actors selling gold-plated wooden nickels on TV to pawn shops hiring street dancers dressed as clowns waving signs saying "We buy gold" at passing cars. Speculators piled in for fast money and fund managers were backed against a wall with clients demanding their portfolios be chock full of gold. Of course, when those same clients wanted to get money out of the market still feeling the sting of '08, gold was summarily unloaded to cover redemptions.
Until 2005, gold was actually not that easy to own. Investors had to purchase physical gold in bars or coins and then have a secure place to store it. Gold futures were virtually in accessible to the average investor. Enter the world of ETFs. Investors could buy and store physical gold at their fingertips on the keyboard with GLD. As this developed, investors could even buy leveraged gold positions and gold futures. As "what goes up, must come down", inverse ETFs effectively shorting gold came into existence.
Bear in mind, regardless of an ETFs holdings, they are, in fact, equities bought and sold on the open market just like any other stock.