If you invest in gold as of today, What is the forecast price in 10 to 20 years?

Not everyone invests in gold. As soon as the panic passes and the stock market goes up slightly, gold will fall.

Gold rises and falls in cycles,  

when people are nervous. But it can be more valuable with time. Twenty years from now, if people are concerned, it will fall. If people are more interested in stocks will be down. But it grows over time because gold does not grow. If you buy a company and hold it for 20 years, interest in the shares going up and down (like gold - but cyclkes opposite), but the company will grow and have more value.

Consider this

With gold, the fundamental investment (gold itself) does not grow. If you have an ounce of gold will always be an ounce.