Why doesn't this GUI work and also it just keeps repeating ?

Looks like you don't really understand how events are working. "Connect" connects a function to an event. The function (usually people call it an event listener) will run every time an event happens, and it will run parallel to main script and all other event listeners. These event listeners are not synced with eachother. That's why your debounce doesn't work. You should put it inside the function.

mouse.Button1Up:Connect(function()
    if cando and db then
        cando = false
        GUI.Time.Visible = true
        wait(1)
        GUI.Time.Text = "4"
        wait(1)
        GUI.Time.Text = "3"
        wait(1)
        GUI.Time.Text = "2"
        wait(1)
        GUI.Time.Text = "1"
        wait(1)
        GUI.Time.Visible = false
        cando = true
        db = false
    end
end)

The second problem is that it is possible to attach multiple functions to a single event type, and you are creating a listener (the buttondown one) every time you equip the tool, and this listener creates another one every time you press a button. If you play for too long you'll end up with a lot of listeners processing every your click. This will lead to a memory leak and performance issues. Make sure that you create them once or destroy them afterwards.

local cando = true
local tool = script.Parent
local player = game:GetService("Players").LocalPlayer
local oGUI = script:FindFirstChild("ScreenGui")
local db = false

--save the listeners to destroy them later
local shoot_listener, reload_listener

tool.Equipped:Connect(function(mouse)
    local GUI = oGUI:Clone()
    GUI.Parent = player.PlayerGui
    wait(1)
    GUI.Equiping.Visible = false
    shoot_listener = mouse.Button1Down:connect(function()
        if not db == true then
            db = true
            local ray = Ray.new(tool.Glowstone.CFrame.p, (mouse.Hit.p - tool.Glowstone.CFrame.p).unit * 400)
            local part, position = workspace:FindPartOnRay(ray, player.Character, false, true)
            local beam = Instance.new("Part",workspace)
            beam.FormFactor = "Custom"
            beam.Material = "Ice"
            beam.BrickColor = BrickColor.new("Cyan")
            beam.CanCollide = false
            beam.Anchored = true
            beam.Transparency = 0.15
            beam.Locked = true
            local distance = (tool.Glowstone.CFrame.p - position).magnitude
            beam.Size = Vector3.new(0.3 , 0.3 , distance)
            beam.CFrame = CFrame.new(tool.Glowstone.CFrame.p, position) * CFrame.new(0,0, -distance /2)
            game:GetService("Debris"):AddItem(beam, 0.1)
            if part then
                local human = part.Parent:FindFirstChild("Humanoid")
                if not human then
                    human = part.Parent.Parent:FindFirstChild("Humanoid")
                end
                if human then
                   human:TakeDamage(3)
                   human.WalkSpeed = 1
                end
            end
             end
           end
    end)

    reload_listener = mouse.Button1Up:Connect(function()
        if cando and db then
            cando = false
            GUI.Time.Visible = true
            wait(1)
            GUI.Time.Text = "4"
            wait(1)
            GUI.Time.Text = "3"
            wait(1)
            GUI.Time.Text = "2"
            wait(1)
            GUI.Time.Text = "1"
            wait(1)
            GUI.Time.Visible = false
            cando = true
            db = false
        end
    end)
end)

tool.Unequipped:Connect(function()
    --destroy the listeners
    shoot_listener:disconnect()
    reload_listener:disconnect()
end)