You should allocate all 4 choices (listed below) due to the following reasons and risks.
1. Physical Gold
Insurance against a true breakdown of the system when financial / commodity markets aren't functional.
Risk - It's more insurance & not Investment.
2. Sovereign Gold Bonds
Very attractive due to 2.5% p.a. coupon reflects the price of gold and cap gains tax-free post 8-year holding.
Risk - long cycle. If gold prices rise and then fall again by the time your 8 years are over.
3. Gold ETFs
Direct and easily accessible play on gold prices without needing to buy and store physical gold.
Risk - At higher gold values, availability of physical, costs of insuring it, protecting it, etc become higher and likely reflect in TERS
4. Gold Mining cos
Reason-Profits jump disproportionately with rising Gold prices as costs do not rise as fast thereby expanding margins.
Risks - At higher gold prices, governments can appropriate mines or bring in other measures to get their share. Very high beta. You need to time exit.