How do you make a Gui appear when a player touches a block?

I've been needing a code for a while for this. Here's the link to the place that I'm about to reference to. Okay, so just join the game so you can understand. See translucent green cylinder going into the sky? I need it to show a new mission in the top left corner of your screen.

local Gui = -- Put Variable here--
local brick = --Put Variable Here--
local humanoid = game.WorkSpace.Players.FindFirstChild("Humanoid")
function onTouched(brick)
if Humanoid true then
Gui.Visible = true
wait()
end

brick.Touched:connect(OnTouched)