Juan Carlos Artigas, ?Director of Investment Research at the WGC, told MINING.com, the continued strong investment was  fuelled by increasing global economic and political uncertainty which was only exacerbated by the unexpected outcome of the UK referendum, looming elections in the US.
Worries about the impact of negative interest rates, ultra-loose monetary policies in many developed economies and the delays to the normalization of the interest rate regime in the US also drove investors to gold.

"Pent up demand among investors who have been looking for a way back in, found the necessary catalyst"

Artigas said the dollars flowing into investment products weren't necessarily "new money"  but much of it represented asset managers simply increasing existing allocations or re-entering the gold market. He believes these ETF investment are likely to be "sticky":

‘While there is some evidence of momentum buying, investors are primarily initiating or rebuilding strategic, long-term holdings after the wash-out of positions since early 2013.

"Pent up demand among investors who have been looking for a way back in, found the necessary catalyst to do so this year."

Artigas said gold ETF inflows are split about half-half among retail and institutional investors. In that sense ETF investment is "democratic" – whether you're an individual or large-scale hedge fund you pay the same price and are charged the same fees

Eoin Treacy's view

No one knows what the eventual repercussions of negative interest rates or even helicopter money are likely to be but it appears sensible to have at least some form of hedge against an unruly outcome. 

So far we have evidence that quantitative easing is not particularly effective at boosting economic growth but it does wonders for asset prices namely stocks, bonds and property. Gold tends to perform more as a hedge against the debasement of currency on a grand scale or perhaps more accurately against government manipulation of markets. Therefore there is a clear argument for having at least some gold in one’s portfolio considering the political and monetary environment we now reside in.  

Total Known ETF Holdings of Gold hit a medium-term low at the end of 2015. It continues to rebound and that helps to confirm the trend of additional investment demand for the metal even after prices have jumped from their lows. 

Gold had developed a short-term overbought condition by the time it encountered resistance below $1400 in early July and has been engaged in a consolidation since, which has allowed the overextension relative to the 200-day MA to be partially unwound. In the absence of a sustained move below the trend mean, there is no reason to suspect this range is anything other than a pause in an otherwise medium-term bull market environment.