How do I make a player has joined/left gui?

Well either you look it up on youtube or it's like this i'll give you an example:

Join example:

game.Players.ChildAdded:Connect(function(plr)
    if plr:IsA("Player") then
    print(plr.Name.." joinned the game")
    end
end)

Leave example:

game.Players.ChildRemoved:Connect(function(plr)
    if plr:IsA("Player") then
    print(plr.Name.." left the game")
    end
end)

OR

game.Players.PlayerAdded:Connect(function(w)
    print(w.." joinned the game")
end)

game.Players.PlayerRemoving:Connect(function(w)
    print(w.." left the game")
end)