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.

Easy, just check if it's nil using an If Statement.

For example:

if game.Workspace.Player1 then --If you just leave it like that then it just sees if the Player2 is true.
    print("Player1 is here!")
end

if game.Workspace.Player2 == nil then --Oh, this is different. you can also use "== nil" to see if it exists.
    print("Player2 is here also!")
end

if game.Workspace.Player3 ~= nil then --Using "==" and "~=" do basicly the same thing.
    print("Player3 has joined the game!")
end

Now to add it to the script.

function intermission()
        for z = 45,1, -1 do
            wait(1)
            for i,v in pairs(game.Players:GetPlayers()) do
                GUI1 = v.PlayerGui.Intermission.Frame
                GUI2 = v.PlayerGui.NewsBar.Frame
                MapName = v.PlayerGui.StartRound.Frame.Map
                GUI3 = GUI1.Title
                Time = GUI1.Seconds


                GUI1.Visible = true
                GUI2.Visible = true
                Time.Text = z

                if z == 1 then 
                    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()
                    GUI3.Text = "Loading Map..."
                    Time.Text = " "
                    MapName.Text = t.Name
                    wait(7)
                    TeleportToMap()
        elseif workspace["Abandoned Church"] or workspace.Campsite or workspace.Facility then return end

                end

            end
        end

    end