It's called banding and a normal artifact. There are not enough steps to create a smooth transition and thus rows of pixels are duplicated, causing this stairstepping. To minimize it, check the dithering options for the brushes or apply a Blur filter with dithering enabled. Also consider working in 16bit or 32bit. This may have repercussions, though, when you need to convert it back e.g. for output to a web format and the issue may come back.