Not that there is such a thing as "fake gold" but there are plenty of counterfeits made to pass off as gold.
Here's a great example:
Notice the very suspicious thickness of the bar itself outside of the holder. While the bar does weight 20 grams, it is definitely not the proper dimension for gold.
The Sigma Metalytics tester does not like it.
And the X-ray spectrometer reveals the truth.
Be careful out there! There are many other examples of this type of crap, even some really good counterfeit Krugerrands!