How do I go about making this "Life Counter" GUI?

Well, there are a couple of problems that you currently have in your code:

• referencing the startergui instead of the playergui

• trying to access the server storage from a local script

• trying to change a value and have the change show up on an object

With that said, we can get to fixing your code

With the first problem, instead of referencing the StarterGui, we need to reference the playerGui which is a child of the player object so we can get it by doing:

local plrgui = game.Players.LocalPlayer.PlayerGui

Now for the second problem, you can circumvent this by simply moving the objects in question from ReplicatedStorage . However, as I don't really know what you are trying to accomplish, but I'd like to leave a warning that setting the position of anything in a local script doesn't change it for the server.

With that said, here is the fixed code:

local plr = game.Players.LocalPlayer
local rep = game.ReplicatedStorage
local plrgui = plr.PlayerGui
script.Parent.Humanoid.Died:Connect(function(dead)
	local Life = plrgui.Lives.CurrentLives
	local Text = plrgui.Lives.TextLabel
	Life.Value = Life.Value - 1
	Text.Text = "X " ..Life.Value
	print(Life)
	if Life.Value < 0 then
		rep.Magna.PrimaryPart.Position = Vector3.new(-36.176, 150.398, 851.044)
		Life.Value = 3
	end
end)

Addition: I realized that I forgot to explain the 3rd problem originally, so here it is:

What you are by defining life and text like that makes them fixed values instead of references Thus any change to those variable doesn't show up on the textlabel or int value.

I believe incapaxx's answer here provides a great explanation on the topic

Hopefully this helped, have a nice day!