You can find the answer to your question in the support ticket that you have opened for the same topic.
I will give the answer here as well in case there are others interested in the case:
The Gradient style that you get comes from the GridHeaderCellElement which is painted over the row. So, in order to get a Solid style, you should set the GradientStyle of the GridHeaderCellElement to Solid as shown on the screenshot.
the Telerik team
Q3’10 SP1 of RadControls for WinForms is available for download
; also available is the Q1'11 Roadmap
for Telerik Windows Forms controls.