How do I make a gui appear when a tool is equipped ?

use the .Equipped and Unequipped events of a tool. Then, make the gui visible

EXMAPLE

Tool = game.StarterGear.Tool
Gui= game.Players.LocalPlayer.PlayerGui.ScreenGui.Frame

Tool.Equipped:connect(function()
Gui.Visible = true
end)

-- Allows GUI to go away
Tool.Unequipped:connect(function()
Gui.Visible = false
end)

For more information, Use the rblox wiki and go to this link http://wiki.roblox.com/index.php?title=API:Class/Tool

And look at the events section

So I have just an example, I actually use this for my Mobile Controls Basketball.

local plr = game.Players.LocalPlayer --This is a Local Script
local playerGui = plr:WaitForChild("PlayerGui") --Better like this.
local controlGuiClass = script.Parent.ControlsGUI
local tool=script.Parent

--What I did was put the GUI inside the Tool.

tool.Equipped:connect(function()
    local CGClone = controlGuiClass:Clone() 
    CGClone.Parent = playerGui --Transfer the Cloned GUI into the Player's GUI.
    CGClone.TextButton.Visible = true --I have my Visibility turned off, so I manually turn it on in this script.
end)

tool.Unequipped:Connect(function()
    if playerGui:FindFirstChild("ControlsGUI") then -- B/c it'll check the original variable, which you'll have to have the script
        playerGui.ControlsGUI:Destroy()
    end
end)

--Whenever you Unequip that tool, the GUI would be destroyed, meaning if you didn't have the Unequipped function, you would have a GUI still on your screen lol.

Also you could put it inside the Replicated Storage, but this method up here is more efficient, at least that's what I think.

Hopefully this helps, LukeGabrieI