I have not found a method in System.Drawing on how to draw a gradient.
Gradient brush
I`m so sorry but in System.Drawing I not found GradientBrush - only SolidBrush
GHIElectronics.TinyCLR.UI.Media.LinearGradientBrush
Also, System.Drawing.DrawRectangle and DrawEllipse have gradient arguments.
This is what I found in the 2.1.0-rc1 code though it has probably been there a while