One tonne of gold: $38,600,000
One Bitcoin: $4,100
We should compare market caps. Anything else is irrelevant. Market cap itself is not the most reliable data but that's the best we got.
In Oct 2016;
http://www.zerohedge.com/news/2016-10-27/needham-raises-bitcoin-price-target-848-heres-why
Now;
Conclusion; bitcoin has %0.87 of the total gold market. If an ounce of gold is $1270 while the market cap is $8,000 billion, that means the total supply is: 6,299,212,598 ounces. So;
An ounce of gold from 6,299,212,598 ounces = 1270$
A btc from 16,507,962 bitcoins = 4100$
16507692/6299212598= 0.0026206BTC
If 0.0026206BTC ever equals to 1270$, that will be the time when bitcoin will become as valuable as gold.
When that time comes 1bitcoin will be = 500k$
so, to defeat gold, 1btc should be 500k$ or more. Btc and gold supply both will rise in time but i am not crazy enough to calculate those.