The SPDR's holdings have fallen nearly 25 metric tons so far in July, putting them on track for their biggest monthly outflow since December.
Demand has tailed off as gold prices have wilted against a backdrop of dollar strength and soft physical demand. They are currently down 1.6 percent this month at $1,572.
Gold ETFs, which issue securities backed by physical bullion, have proved a popular way to invest in gold since they first appeared in 2003, and now hold around 70 million ounces, or more than 2,000 metric tons, of metal.
It's an important segment of the gold market, and arguably it's been a fantastic vehicle to get some of the smaller investors and some of the institutions into investing in gold, Societe Generale analyst Robin Bhar said.
It's worked its magic, but on the margins, those who've used ETFs more as a leverage on the price may start to cash in. You could see some more outflows, though I don't see a complete reversal.
The SPDR's bullion holdings, at 1,254.6 metric tons, are larger than those of Switzerland, Russia and Japan. They peaked at over 1,320 metric tons in June 2010, as investors scurried for the safety of physical gold as the financial crisis bit.
Holdings of gold exchange-traded products are easing overall, with an index of ETPs tracked by Reuters, including those operated by iShares, Zurich Cantonalbank and ETF Securities, showing an outflow this month after rising 1.6 percent in June.
Overall holdings remain relatively resilient, Barclays Capital said in a note.