How do you check to see if something exists?

Normally, when you have objects, you check if they have something by just use [] or .:

if string.sub then
	-- There is a `sub` in the `string` library!
end

if string["sub"] ~= nil then
	-- This is the same
end

if not string.sub == nil then
	-- This is the same, too
end

However, for ROBLOX objects, if the object isn't there, you will get an error, not nil.

On a ROBLOX object, instead use :FindFirstChild:

if workspace:FindFirstChild("BlueTaslem") then
	-- It didn't return `nil`, so there is a BlueTaslem in the workspace
end

Nonetheless, Instead of applying this, just fix your script! Don't cover up problems, solve them! That code shouldn't be happening over and over -- that is the problem.

First: checking if z == 1 isn't the wise way to do it. Instead, just put that code after the loop -- what you mean by if z == 1 is that the loop is done, so use that fact!

The only issue is MapName -- which you'll just add another short loop to do. However, this is not the best way to structure this, and, for instance, won't work with FilteringEnabled.

function intermission()
	for time = 45,1, -1 do
		wait(1)
		for _, player in pairs(game.Players:GetPlayers()) do
			local GUI1 = player.PlayerGui.Intermission.Frame
			local GUI2 = player.PlayerGui.NewsBar.Frame
			local Time = GUI1.Seconds
			GUI1.Visible = true
			GUI2.Visible = true
			Time.Text = time
		end
		num = 2
		maps = {"Abandoned Church", "Campsite", "Facility"}
		m = math.random(1, num)
		t = game.Lighting:FindFirstChild(maps[m]):clone()
		t.Parent = game.Workspace
		t:MakeJoints()
		for _, player in pairs(game.Players:GetPlayers()) do
			local MapName = player.PlayerGui.StartRound.Frame.Map
			local GUI3 = GUI1.Title
			GUI3.Text = "Loading Map..."
			Time.Text = " "
			MapName.Text = t.Name
		end
		wait(7)
		TeleportToMap()
	end
end

A more correct way to manage GUIs would be something like this:

-- A LOCALSCRIPT in the PlayerGui (i.e., put it in the StarterGui)
local player = game.Players.LocalPlayer
local gui = player:WaitForChild("PlayerGui")
local intermissionFrame = gui.Intermission.Frame
local newsBar = gui.NewsBar.Frame

local globalMessage = game.ReplicatedStorage.Status
local globalInt = game.ReplicatedStorage.InIntermission

function update()
	local msg = globalMessage.Value
	if globalInt.Value then
		-- In intermission
		newsBar.Visible = true
		intermissionFrame.Visible = true;
		intermissionFrame.Seconds.Text = msg
	else
		intermissionFrame.Visible = false
		newsBar.Visible = false
		intermissionFrame.Visible = false
		gui.StartRound.Frame.Map.Text = msg
	end
end

globalMessage.Changed:connect(update)
globalInt.Changed:connect(update)
update()

Our game round manager is now much simpler:

function intermission()
	for time = 45,1, -1 do
		wait(1)
		game.ReplicatedStorage.InIntermission.Value = true
		game.ReplicatedStorage.Status.Value = time
	end
	local maps = {"Abandoned Church", "Campsite", "Facility"}
	local m = math.random(1, #maps)
	local t = game.Lighting:FindFirstChild(maps[m]):Clone()
	t.Parent = game.Workspace
	t:MakeJoints()
	game.ReplicatedStorage.InIntermission.Value = false
	game.ReplicatedStorage.Status.Value = "Loading Map..."
	wait(7)
	TeleportToMap()
end

where InIntermission is a BoolValue in ReplicatedStorage and Status is a StringValue in ReplicatedStorage.