--Fixed, look at second answer--I'm trying to get a GUI script to work with two scripts....?

On your first script, everything is fine except for line 11. Instead of ":FireClient()", you are supposed to use ":FireAllClients()". Though this fires all clients instead of just one, you can always check whether you are the player who died using the local script since you are sending back the "player" variable through the ":FireAllCients()".

Here is your edited script:

local player = game:GetService('Players').PlayerAdded:Connect(function(player)
    local char = player.CharacterAdded:Connect(function(character)
        local hum = character:WaitForChild("Humanoid")
        local repstorage = game:GetService("ReplicatedStorage")
        local DeathEvent = repstorage:WaitForChild("DeathEvent")

        hum.Died:connect(function()
            print("player has died! detected from server")
            DeathEvent:FireAllClients(player)
        end)
    end)
end)

On your local script, you can just use

local player = game.Players.LocalPlayer

instead, as it is in a local script. Otherwise, it should work.

Here is your edited local script:

local localplayer = game:GetService('Players').LocalPlayer
local char = localplayer.Character
local hum = char:WaitForChild("Humanoid")
local DeathEvent = game.ReplicatedStorage.DeathEvent    
local livestext = "lives: "
local numberoflives = 3
local lives = livestext .. numberoflives
local text = script.Parent.Text

DeathEvent.OnClientEvent:Connect(function(player) --condenses the creation of the function and connects it to the event in the same line
    if player == localplayer then --Check if the player mentioned in the event is the same as the gui owner...
        print("player has died! remote event worked")
        text = "is it working"
    end
end)

Tell me if anything breaks.

@demcode, yes, your version partially worked. to get the text to change upon death I made it like this (local script, the server script was fine)

local localplayer = game:GetService('Players').LocalPlayer
local char = localplayer.Character
local hum = char:WaitForChild("Humanoid")
local DeathEvent = game.ReplicatedStorage.DeathEvent    
local livestext = "lives: "
local NumberOfLives = 3
local livesGui = script.Parent

DeathEvent.OnClientEvent:Connect(function(player) --condenses the creation of the function and connects it to the event in the same line
    if player == localplayer and NumberOfLives > 0 then --Check if the player mentioned in the event is the same as the gui owner...

        livesGui.Text = livestext .. tostring(NumberOfLives - 1)
        NumberOfLives = NumberOfLives - 1

    elseif player == localplayer and NumberOfLives <= 0 then
        livesGui.Text = "you have no more lives"
    end
end)

in my game once the player has no more lives I will teleport them.