The gold block is a decoration block. It can used as a more compact way of storing gold.
Gold apple trees
The gold block can be used to grow golden apple tree. To plant it, place a golden apple seed onto a gold block. After a while it will grow. It cannot be grown instantly with bone meal. It will look like a regular tree, however it will have golden apples in its leaves.
The gold block can be used to store gold ingots.