How to make Gui for the player who Touched it Only?

An easier way to do this is detect via Server-Side then use a RemoteEvent to fire the client.

Like so:

Server Script

-- Put this script inside the part you want touched to activate the UI.
local part = script.Parent
local Players = game:GetService('Players')
local RS = game:GetService('ReplicatedStorage')


part.Touched:Connect(function(otherPart)
	local Player = Players:GetPlayerFromCharacter(otherPart.Parent)
	if Player then
		RS.RemoteEvent:FireClient(Player, 'Stage Two: Water!') -- first parameter being the client we are firing, second being the text
	end
end)

Now, for the local script. Preferably put this inside StarterGui or the UI itself, doesn't matter.

LocalScript

local Player = game.Players.LocalPlayer
local RS = game:GetService('ReplicatedStorage')

function textActivated(textInfo)
	local textlabel = **TextLocationHere**
	if textlabel.Visible == false then
		textlabel.Visible = true
		textlabel.Text = textInfo -- this will be the text you sent over
		wait(2)
		textlabel.Visible = false
	end
end

RS.RemoteEvent.OnClientEvent:Connect(textActivated)

In hindsight, you'll be able to use the same remote for each level change, just by changing the remote parameter to what you desire. The code is somewhat messy and rushed and could probably be a lot shorter but it's early in the morning and I haven't had my tea yet.

Either way, give that a go and let you know if you need any help.