Why do I own gold ? Two reasons :
1. It is a good diversifier. Historically, adding a slice of gold in a stock-heavy allocation produced less volatile portfolios without reducing significantly CAGR (i.e, generated higher SWR).
2. It is an insurance against political/financial risk. Bailouts are a hot topic here in EU, and you can't be sure your financial assets will still be yours in the years to come. That might be less of a risk in other parts of the world (USA, eg), or a bigger risk in countries that are known to play with their citizen's money (Argentina, eg). YMMV.
If you invest for reason 1, you can invest in bullions or ETFs. If you invest for reason 2, the only sane choice is gold bullion.
Now, about your more particular question. Well, gold (somewhat) protects you from a stock market krach, since, more often than not, gold goes upward when stocks go downward. But how can you protect yourself from a gold krach, i.e when the USD is strong ? Well, stocks and bonds.
Don't try to play with reverse ETFs, exchange rates bets and the like. It is not your job. They will bite you.