Unless it stated somewhere in the movie that Gandalf's sword should glow like Sting (which it didn't), I don't think this is a slipup. Although Jackson tried to remain true to Tolkien in as many aspects as he could, there are of course some differences, and this is just one of them.
To my knowledge, Sting is the only weapon that is referrenced by Tolkien that glows blue when Orcs are present. There were three elvish blades that are found in the trolls cave in the hobbit and neither Orcrist or Glamdring are said to glow. Not a slip.