GradientFill uses colors of 64 bits, i.e. 16 bits per component (16 bits each for the red, for the green, for the blue and for the alpha), so the type of R, G and B fields in the TRIVERTEX struct are ushort, and not byte.

[DllImport("gdi32.dll", EntryPoint = "GdiGradientFill", ExactSpelling = true)]

public static extern bool GradientFill(

[DllImport("gdi32.dll", EntryPoint = "GdiGradientFill", ExactSpelling = true)]

public static extern bool GradientFill(

[DllImport("gdi32.dll", EntryPoint = "GdiGradientFill", ExactSpelling = true)]

public static extern bool GradientFill(

Declare Function GradientFill Lib "gdi32.dll" (TODO) As TODO

The GDI32.dll does not contain a GradientFill() function. The actual entry point is GdiGradientFill(). That's why the 'EntryPoint' attribute is present.

Documentation

[GradientFill] on MSDN