How do I make A GUI effect more than the local player? [closed]

You need to use a RemoteEvent to fire a local event to the server. Insert this is the event of the LocalScript (What makes the Gui visible.)

local event = Instance.new("RemoteEvent", game.ReplicatedStorage)
event.Name = "GuiEvent"

--Put this in your function.

    event:FireAllClients()

That's not all. We need to connect the event to the server. Put this in a Script in ServerScriptService.

local gui = "GUI" --Make this the name of your Gui.
local event
local replicatedStorage = game:GetService
local waitEvent = replicatedStorage:WaitForChild("GuiEvent")
local players = game.Players:GetPlayers()

if waitEvent then
    event = waitEvent
end

event.OnClientEvent:Connect(function()
    for _,player in pairs (players) do
        local playerGui = player:WaitForChild("PlayerGui")
        playerGui[gui].Visible = true --Do whatever you want to do to the Gui.
end)

Hope this helps!

Don't forget to accept this answer if it worked!

No, seriously though. I spent a long time writing this.