How do I make a GUI that shows which players survived each round?

There are many ways you can do this, but the best way in lua is to create tables for casualties and tourists, and reset them at the end of every round. In order to do this, you would first need to connect the humanoid of every player to the died function, and add them to casualties, if they are found in tourists (you would also need to remove them from tourists) the script for connecting the humanoid is below:

-- This would be a `Server script` in `ServerScriptService`

local Tourists = {} -- Making the tourist table

if #Tourists == 0 then
    for i, v in pairs(game.Players:GetPlayers()) do --going through all the players
        table.insert(Tourists, v.Name) -- Inserting players name into Tourists
    end
elseif #Tourists > 0 then
    for i, v in pairs(Tourists) do
        table.remove(Tourists[i])
 --[[ Without the line above, on the second round their name would be                                               
        there twice, so we clear the table and then add all the players again.
    ]]
    end
    for i, v in pairs(game.Players:GetPlayers()) do --going through all the players
        table.insert(Tourists, v.Name) -- Inserting players name into Tourists
    end
end

local Casualties = {} -- Creating Casualties table

game.Players.PlayerAdded:Connect(function(plr) -- When a player joins
    plr.CharacterAdded:Connect(function(char) -- When their character loads
        char.Humanoid.Died:Connect(function() -- When they die...
            for i, v in pairs(Tourists) do -- going through all the values in Tourists table
                if v == char.Name then -- Checking if their name is in there
                    table.insert(Casualties, char.Name) -- if it is, add it to casualties
                    table.remove(Tourists[i]) -- Removing their name from tourists
                end
            end 
        end)
    end)
end)

As for making the GUI's at the end of the round, you would have to figure that out yourself, as I can not see your whole game. I wish you luck and hope I helped so far.

-Cmgtotalyawesome