Make GUI OPEN/CLOSE NO X BUTTON

To make it open you to

NameOFBUTTON.MouseButton1Down:connect(function()
NameOFURMAINGUI.Visible = true
NAMEOFOPENBUTTON/OPENFRAME.Visible = false
CLOSEBUTTON.Visible = true
end)

To make it close you to

CLOSEBUTTONNAME.MouseButton1Down:connect(function()
NAMEOFOPENBUTTON.Visible = true
MAINGUINAME.Visible = false
CLOSEOPENBUTTON.Visible = false
end)