Hi Vahid,
You need to use the chart control for this purpose as indeed the gauge only supports two colors per gradient. To replicate the attached picture you can use a bar series with one bar (bar width percent should be set to 100%) and then use the following custom code:

using System;
using System.Drawing;
using Nevron.GraphicsCore;
using Nevron.Chart;
using Nevron.ReportingServices;

namespace MyNamespace
{
 /// <summary>
 /// Sample class
 /// </summary>
 public class MyClass
 {
  /// <summary>
  /// Main entry point
  /// </summary>
  /// <param name="context"></param>
  public static void RSMain(NRSChartCodeContext context)
  {
   NChart chart = context.document.Charts[0];
   NBarSeries bar = (NBarSeries)chart.Series[0];
   
   bar.Palette = new NPalette();
   bar.Palette.SmoothPalette = true;
   bar.Palette.Mode = PaletteMode.Custom;
   bar.Palette.Add(0, Color.Red);
   bar.Palette.Add(0.5, Color.Green);
   bar.Palette.Add(1.0, Color.Blue);
  }
 }
}
It shows how to apply a 3 color gradient to a bar series. Hope this helps - let us know if you meet any problems or have any questions.