@Dealman, so i need put this on my code ? 
@KRZO This is my code for tests. 
Using normal MTA GUI: 
 
 
  
function createMenuGUI() 
local xw, xh = guiGetScreenSize() 
local Width, Height = 473,284 
local X = (xw/2) - (Width/2) 
local Y = (xh/2) - (Height/2) 
     
window_test = guiCreateWindow(X, Y, Width, Height, "Test", false) 
     
btn_test = guiCreateButton(155, 135, 200, 30, "Test Button", false, window_test) 
end 
  
function openGUI() 
    createMenuGUI() 
end 
addEventHandler("onClientResourceStart", getResourceRootElement(getThisResource()), openGUI) 
 
 
Result:  
http://i.imgur.com/111RHhz.png
 
 
 
Using dxGUI: 
 
 
 function createMenuGUI() 
    local xw, xh = guiGetScreenSize() 
    local Width, Height = 473,284 
    local X = (xw/2) - (Width/2) 
    local Y = (xh/2) - (Height/2) 
     
    window_test = exports.dxgui_v1:dxCreateWindow(X, Y, Width, Height, "Test", tocolor(255, 255, 255, 255), "Lighter Black") 
     
    btn_test = exports.dxgui_v1:dxCreateButton(140, 135, 200, 30, "Test Button", tocolor(255,0,0,255), "default-bold", "Orange", window_test) 
end 
  
function openGUI() 
    createMenuGUI() 
end 
addEventHandler("onClientResourceStart", getResourceRootElement(getThisResource()), openGUI) 
 
 
Result (nothing):  
http://i.imgur.com/vH3K4Mq.png