How do you index the Player that touches the brick?

Using the Touched event, you can have this script as well as the gui you want to show in the player.

local part = script.Parent
local gui = part.ScreenGui -- The gui inside the part
local debounce = true

part.Touched:connect(function(hit) -- The Touched event
    if debounce then debounce = false
    local model = hit.Parent
        if game.Players:findFirstChild(model.Name) and model:findFirstChild("Humanoid") then -- Is a player
        local add= gui:Clone()
        add.Parent = game.Players[model.Name].PlayerGui
        end
    wait(3)
    debounce = true 
    end
end)

**EDIT: **If you want to store the player's name, say a table then (I haven't tested this so might not work),

playersTouched = {} -- people who touched the brick.

local part = script.Parent
local gui = part.ScreenGui -- The gui inside the part
local debounce = true

part.Touched:connect(function(hit) -- The Touched event
    if debounce then debounce = false
    local model = hit.Parent
        if game.Players:findFirstChild(model.Name) and model:findFirstChild("Humanoid") then -- Is a player
        table.insert(playersTouched, model.Name)
        local add= gui:Clone()
        add.Parent = game.Players[model.Name].PlayerGui
        end
    wait(3)
    debounce = true 
    end
end)