Yonks ago I read Jim Rodgers book 'Investment Biker' in which he points out the diversifiction benefit of investing in commodities in a portfolio that invests in American equities. You can read up a lot more on the diversification benefits of commodities on the Seeking Alpha web site. So that's why I started investing in commodities. Investing in an EFT that follows an index also allows you to benefit from the 'roll yield' as the returns when commodity contracts are rolled over are independent of market returns. (I did a post on this, providing references to academic research on AAM some time ago.) So you can initially look at commodities solely as a hedge against USD equities.
Then you can get more into them and look at commodities as an asset class in itself. So now you could consider eg an investment trust that invests in commodity producers. Or if you are a stock picker, investment in a gold miner, a diversified metal miner and an oil producer probably has you covered. You can make money here, but it is not without its risks. Then, if you really get into commodities, you could also look at agriculture.
Note, this is not as the OP asked for gambling on commodity stocks. It's long term investment in commodities initially for their diversification benefit, and then as an asset class.