I have this simple test GUI I'm using to learn GUIs, but I'm stuck.
 

Spoiler

package net.insane96mcp.xpholder.gui;

import java.io.IOException;

import org.lwjgl.input.Keyboard;
import org.lwjgl.opengl.GL11;

import net.insane96mcp.xpholder.XpHolder;
import net.minecraft.client.Minecraft;
import net.minecraft.client.gui.GuiButton;
import net.minecraft.client.gui.GuiScreen;
import net.minecraft.util.ResourceLocation;

public class TestGui extends GuiScreen{
	private final int GUI_WIDTH = 64;
	private final int GUI_HEIGHT = 64;
	private ResourceLocation GUI_TEXTURE;
	private GuiButton buttonClose;
	
	public TestGui() {
		GUI_TEXTURE = new ResourceLocation(XpHolder.MOD_ID, "textures/gui/xpholder.png");
		
	}
	
	@Override
	public void initGui() {
        Keyboard.enableRepeatEvents(true);
		buttonClose = new GuiButton(0, width / 2 - 20, (height - GUI_HEIGHT) / 2 - 20, 40, 20, "Close");
		
		buttonList.add(buttonClose);
	}
	
	@Override
	public void drawScreen(int mouseX, int mouseY, float partialTicks) {
		GL11.glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
		Minecraft minecraft = Minecraft.getMinecraft();
		minecraft.getTextureManager().bindTexture(GUI_TEXTURE);
		int offsetScreenLeft = (width - GUI_WIDTH) / 2;
		int offsetScreenTop = (height - GUI_HEIGHT) / 2;
		//System.out.println(width + " " + height + " " + offsetScreenLeft + " " + offsetScreenTop);
		drawTexturedModalRect(offsetScreenLeft, offsetScreenTop, 0, 0, GUI_WIDTH, GUI_HEIGHT);
		
		super.drawScreen(mouseX, mouseY, partialTicks);
	}
	
	@Override
	protected void actionPerformed(GuiButton button) throws IOException {
		if (button.equals(buttonClose))
		mc.displayGuiScreen((GuiScreen)null);
		super.actionPerformed(button);
	}
	
    @Override
    public boolean doesGuiPauseGame()
    {
        return true;
    }
}

 

When the gui is opened it shows only 1/4th of the texture
The texture is this one (128x128):

I get this in-game

I really don't know if I'm missing something.

Minecraft assumes that all gui textures are 256x256 unless otherwise specified.

 

64/256 = 0.25