You can achieve this with a custom renderer.
public class CustomToolStripRenderer : ToolStripProfessionalRenderer
{
public CustomToolStripRenderer() { }
protected override void OnRenderToolStripBackground(ToolStripRenderEventArgs e)
{
//you may want to change this based on the toolstrip's dock or layout style
LinearGradientMode mode = LinearGradientMode.Horizontal;
using (LinearGradientBrush b = new LinearGradientBrush(e.AffectedBounds, ColorTable.MenuStripGradientBegin, ColorTable.MenuStripGradientEnd, mode))
{
e.Graphics.FillRectangle(b, e.AffectedBounds);
}
}
}
Then set your toolstrip to use an instance of this renderer.
public Form1()
{
InitializeComponent();
CustomToolStripRenderer r = new CustomToolStripRenderer();
r.RoundedEdges = false;
toolStrip1.Renderer = r;
}