Einar,

The designer has no default UITypeEditor means for editing a Brush property so you would need to implement this and the necessary TypeConverter. You can cache the brush but need to create a default instance in your constructor, however if you are using this as the basis for end-user colour selection then use a Color instead and wrap your brush creation in your OnPaint inside a Using block.

using (LinearGradientBrush brush1 = new LinearGradientBrush(GradientRectangle, StartColor, EndColor, 90f))
{
   e.Graphics.FillRectangle(brush1, GradientRectangle);
}

The .NET Brush is a wrapper around a GDI/GDI+ unmanaged resource and really should be treated as something in limited supply. By applying the creation of the brush inside the using statement you will get the benefit of having code generated by the compiler to automatically call Dispose on classes that implement the IDisposable interface to release any unmanaged resources inside a try...catch block.

Regards
Neil