I want to draw a Rectangle with color Gradient in GtkGLArea. I know it can be done with Cairo on GtkDrawingArea. But I want to draw it with OpenGL. Can anyone tell me how this can be done?

#include <epoxy/gl.h>
#include <epoxy/glx.h>
#include <gtk/gtk.h>

static void on_realize(GtkGLArea *area) {
  // Debug Message
  g_print("on realizen");
  gtk_gl_area_make_current(area);
  if(gtk_gl_area_get_error(area) != NULL) {
     fprintf(stderr, "Unknown errorn");
     return;
  }
}
static void on_render(GtkGLArea *area, GdkGLContext *context) {
  g_print("on rendern");
  glEnable(GL_SCISSOR_TEST);
  glScissor(50, 50, 200, 200);
  glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glClear(GL_COLOR_BUFFER_BIT);
}

static void
activate (GtkApplication* app,
          gpointer        user_data)
{
  GtkWidget *window;
  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "xx");
  gtk_window_set_default_size (GTK_WINDOW (window), 415, 300);

  GtkWidget *area = gtk_gl_area_new ();
  gtk_window_set_child ((GtkWindow   *)window, area);
  g_signal_connect (area, "realize", G_CALLBACK(on_realize), NULL);
  g_signal_connect (area, "render", G_CALLBACK (on_render), NULL);

  gtk_window_present (GTK_WINDOW (window));
}

int
main (int    argc,
      char **argv)
{
  GtkApplication *app;
  int status;
  app = gtk_application_new ("org.gtk.example", G_APPLICATION_DEFAULT_FLAGS); 
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);
  return status;
}

This image is taken from the GtkColorChooser widget. I trying to draw a rectangle like this picture (Colors can be changed) —