How to access a GUI in a script

Using the code you have, you would access the player's PlayerGui simply by going player.PlayerGui.MYGUI

"And plus, is there a way to make it only once, or once in a certain amount of time?"

Edit: Yes, indeed there is. In order to do this, you'll need to use some type of object that is to be looked for when the event fires. In this case, I'm using a IntValue.

local toolname = "Luger, Colt 45"
script.Parent.Touched:connect( function(hit) 
    if hit.Parent then 
        local player = Game.Players:GetPlayerFromCharacter(hit.Parent) 
        if player and not player:FindFirstChild("No") then 
            if player.Character:FindFirstChild(toolname) then 
                local gui = player.PlayerGui.MyGUI
                local no = Instance.new("IntValue", player)
                no.Name = "No"
                Spawn(function()
                    wait(10) -- Player can only use the button once every 10 seconds
                    no:Destroy()
                end)
            end 
        end 
    end 
end) 

Line 10 basically allows the code to continue running while the scope (the function) runs at it's own pace. You can get this effect a variety of ways, using game.Debris:AddItem(no, 10) or by using the delay function.

Good luck!

Put at the top

debounce = false

In the script.

If debounce == false then 
debounce = true

To access the GUI. Do:

player.PlayerGui.GUINAME

At the end, put

debounce = false