/**
 * @param color1 color in hex.
 * @param color2 color in hex.
 */
 function drawSimpleVerticalGradientRect(x, y, x2, y2, color1, color2) {
    GL11.glBegin(GL11.GL_QUADS);
    glColor(color1);
    GL11.glVertex2d(x, y);
    GL11.glVertex2d(x, y2);
    glColor(color2);
    GL11.glVertex2d(x2, y2);
    GL11.glVertex2d(x2, y);
    GL11.glEnd();
}

/**
 * @param color A color in hex.
 */
function glColor(color) {
    var a = (color >> 24) & 0xFF;
    var r = (color >> 16) & 0xFF;
    var g = (color >> 8) & 0xFF;
    var b = (color) & 0xFF;
    GL11.glColor4f(r / 255, g / 255, b / 255, a / 255);
}

I would like to explain this, but i don't know if i could do it well

Example Usage:

GlStateManager.disableTexture2D();
GlStateManager.enableBlend();
GlStateManager.blendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
// Tells GL to transition between colors
GlStateManager.shadeModel(GL11.GL_SMOOTH);
drawSimpleVerticalGradientRect(10, 10, 390, 390, 0xffffffff, 0x00ffffff);
// Tells GL to use the ONLY' last color
GlStateManager.shadeModel(GL11.GL_FLAT);
GlStateManager.disableBlend();

render2d:

Result:

the easiest way would be to use Core's Reflector

RGui = new Reflector(Gui) 

onrender:

RGui.drawGradientRect(10, 10, 10 + 50, 10 + 50, Color.RED.getRGB(), Color.BLUE.getRGB())

also what about looking into logs or https://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Method.html

What's wrong here?

its protected i cant access it

reflection