How can I make a GUI appear for everyone in a server?

As you yourself said, this can be done with :FireAllClients(), if you want to learn more about it you can have a look here: Roblox Wiki Page - Fire All Clients

For this you also need to know a bit about Server Event. You can find this in roblox wikia: Roblox Wiki Page - OnServerEvent

You can also learn more about RemoteEvents, just click here: Roblox Wiki Page - RemoteEvents

You must first use a RemoteEvent

Create a RemoteEvent and put on ReplicatedStorage, name this as Event (You can choose the name, but if you do not want to edit the script you can leave it with that name that I said.)

Create a Script (Server Script) in ServerScriptService And put this code:

local event = game.ReplicatedStorage.Event -- Location of event

event.OnServerEvent:Connect(function(plr,name) -- On fireserver get player and whitelist name.
    if plr.Name == (name) then -- If player name is in whitelist names 
        game.ReplicatedStorage.Event:FireAllClients(plr); -- Fire all clients.
    else
        plr:Kick("Do not use exploits!")
    end
end)

Now create a LocalScript And put LocalScript in your button. and put this code:

-- Local Script --

repeat wait() until game.Players.LocalPlayer -- Not return nil for Player (Works in LocalScript.)

local whitelistname = {"Avia_Robby"} -- Put your name here
local Player = game.Players.LocalPlayer -- Get Current player
local client = game:GetService("ReplicatedStorage").Event -- Get Event

client.OnClientEvent:Connect(function(plr) -- On fire all clients
    for i,v in pairs(game:GetService("Players"):GetPlayers()) do -- Get all players
        script.Parent.Parent.Parent.NoGame.Visible = false -- Set NoGame for visible false
        script.Parent.Parent.Parent.Waiting.Visible = true -- Set Waiting for visible true
    end
end)

script.Parent.MouseButton1Click:Connect(function() -- On click with button
    for i,v in pairs(whitelistname) do -- Get whitelist names
        if Player.Name == v then -- If player name are in whitelist
            client:FireServer(v); -- Fire Server
        end
    end
end)

:FireAllClients() can only be used by the server, then I made a connection with the server, and the server gives :FireAllClients() and all players will receive the GUI, Why will they receive the GUI? Because when giving FireAllClients you have access to all items of the PlayerGUI because the Client have the GUI's then you have access to everything and can change things.

Not works? tell-me on comments

Good luck in your game! :)

Here's how you would do this.

Option 1:

First make a RemoteEvent and place it in ReplicatedStorage. From there you will need two scripts.

1. A LocalScript inside your GUI or somewhere on the client

2. A Server Script that will be firing at the clients.

ServerScript

game.ReplicatedStorage.RemoteEvent:FireAllClients()

LocalScript

game.ReplicatedStorage.RemoteEvent.OnServerEvent:Connect(function()
    script.Parent.Parent.Parent.NoGame.Visible = false
    script.Parent.Parent.Parent.Waiting.Visible = true
end)

The issue with this method is that based on your needs, it seems that you would also need a RemoteEvent that is called when the player presses the GUI button you seem to have. Sooooooo... Here is another way that some devs may find not as good but I think works fine.

Option 2:

1. Make a RemoteEvent that is fired when the player clicks the GUI button.

2. When fired loop through all the players and enable the GUI (assuming the players already have the gui object in their PlayerGui folder)

for i,player in pairs(game.Players:GetPlayers()) do
    player.PlayerGui.NameOfGui.Enabled = true
end

I hope this helped, if you liked the answer please give me some rep by upvoting and checking it off as correct.