Gui disapear?/ Force Gui

How do i do so, when it hits 0, the gui will auto disapear.

Gui = script.Parent

WG = game.StarterGui.Health.load

X = Gui.Size

HP = 500

while HP > 0 do

    wait()
    HP = HP -10
    Gui.Size = UDim2.new(0,math.floor(HP),0,25)
    print(HP)

end

how to do so when i press the "Dirt" the gui will pop up

local Box = script.Parent --Fortæller hvad vi leder efter
local Grass = Box.grass
local jord = Box.dirt


Box.grass.ClickDetector.MouseClick:connect(function(player)
    game.StarterGui.HealthGrass.Screen.Visible = true
    jord.Decal1.Texture = "http://www.roblox.com/asset/?id=48336811"
    jord.Decal2.Texture = "http://www.roblox.com/asset/?id=48336811"
    jord.Decal3.Texture = "http://www.roblox.com/asset/?id=48336811"
    jord.Decal4.Texture = "http://www.roblox.com/asset/?id=48336811"
    wait(2)
    Box:Destroy()
    Grass:Destroy()
end)

--Helping notes

Box.dirt.ClickDetector.MouseClick:connect(function(player)
    game.StarterGui.HealthGrass.Screen.Visible = true
    jord.Decal1.Texture = "http://www.roblox.com/asset/?id=48336811"  --This Changes the DECAL FROM HERE
    jord.Decal2.Texture = "http://www.roblox.com/asset/?id=48336811"
    jord.Decal3.Texture = "http://www.roblox.com/asset/?id=48336811"
    jord.Decal4.Texture = "http://www.roblox.com/asset/?id=48336811"  -- Stops changeing the DECAL HERE
    wait(2) -- Time before the block disapear
    Box:Destroy() -- Destroys the block(Dirt)
    Grass:Destroy() -- Destroy the block(Grass)
end)