The simple answer here is: all of them. However, if you are expressly wondering about solid pieces / nuggets / flakes / veins of gold, you will find most of that in the lithosphere, because that is a solid layer - and also the only layer we can truly access.
Gold is not a very common mineral, actually. Still, trace amounts of gold can be found in magma, in sea water, even in river beds.
How much gold is in layers other than the lithosphere is conjecture, but our current understanding of the early earth says that when the earth was more uniformly molten the "heavier" minerals "sank" into the center (a process called differentiation). Mostly this is iron, but it is entirely plausible that whatever gold was around at that time may well have "sunk into the core" as well, due to its high specific gravity (this means that it is heavy relative to other minerals).