I honestly have no clue how. The area is a part, where when only one player left is in it then the script will show a gui.
Simple, we will use GetTouchingParts.
example of use:
local onlyPlayer = false -- only one player bool value for i,v in pairs(script.Parent:GetTouchingParts()) do if i == 1 then onlyPlayer = true else onlyPlayer = false end end