The FlexShares Morningstar Global Upstream Natural Resources Index ETF (ticker: GUNR) has more than $5.8 billion in assets and has returned more than 20% over the past year. It counts Exxon Mobil, Monsanto and Rio Tinto among its holdings. Bloomberg's Scarlet Fu and Eric Balchunas talk with Darek Wojnar, who heads up FlexShares ETFs at Northern Trust Asset Management, about GUNR, inflation ETFs and the growth of his firm's ETF business. (Source: Bloomberg)