Tested and works perfectly.
HInv_Window = guiCreateWindow(354,252,362,272,"Haushaltsinventar",false)
guiWindowSetSizable(HInv_Window,false)
HInv_LBL_Welcome = guiCreateLabel(34,25,305,35,"Hallo. Das ist dein Haushaltsinventar.\nVon hier aus kannst du Möbel in dein Haus platzieren.",false,HInv_Window)
guiSetFont(HInv_LBL_Welcome,"default-bold-small")
HInv_Grid = guiCreateGridList(38,69,290,157,false,HInv_Window)
guiGridListSetSelectionMode(HInv_Grid,2)
centerWindow(HInv_Window)
hinvName = guiGridListAddColumn(HInv_Grid,"Name",2)
HInv_BTN_Platzieren = guiCreateButton(90,231,186,22,"Platzieren",false,HInv_Window)
function textITT()
outputChatBox("TEXT")
end
addEventHandler( "onClientGUIClick" ,root,
function()
if (source == HInv_BTN_Platzieren) then
guiSetVisible ( HInv_Window, false )
textITT()
end
end)