Power spectral density: where does the difference from decibel and regular representation come from?
If you zoom in on the linear curve, you would see that it's also "gradually decreasing", just much steeper, than the logarithmic curve. The logarithm simply makes it more visible.
And, yes, what you see is indeed spectral leakage. It can be reduced by using a non-trivial window function as opposed to the rectangular window being employed by default in the periodogram function. Try using a Hann window, that should reduce spectral leakage.