How is gold formed in the earth?

It isn't

Gold is formed in the heart of supernovae as part of the life cycle of a star. A star's life can be divided roughly into three stages;

1 Pre Main sequence, this is when a star is still forming from its cloud of dust and gas.

2 Main sequence this is when the nascent star has sucked in enough matter in the form of dust and hydrogen gas that its mass gets high enough to sustain hydrogen fusion and this it does fusing hydrogen and creating helium as a waste product.

3 Final stage, once the star has used all its hydrogen it briefly stops, collapses and with the raised temperature and pressure it can now start using helium. This will go on in a process called nucleosynthesis and will go down the periodic table using progressively heavier elements that were created by the previous element being fused. This goes on until you reach iron, at that point fusion stops and gravity wins crunching the star down to the point that it explodes (supernova) in the process ALL the elements heavier than iron are created only to be blown out into the interstellar medium.

So how did gold get to Earth? When our solar system formed , it formed in a region that had recently had a supernova so our cloud of dust and gas and rocks had gold platinum, copper uranium and all the other useful heavy elements. When earth formed it formed in the right zone to collect heavy minerals.