As the op seems to be from Germany, I wanted to add that forbidding your employees to discuss their salaries most likely won´t stand before court in Germany.

[Here is a reference ruling][1]

Not this was not tried before the supreme court, but the employer would have to have really strong arguments why wages would have to be a secret to get a ruling in his favor.


  [1]: https://dejure.org/dienste/vernetzung/rechtsprechung?Gericht=LAG%20Mecklenburg-Vorpommern&Datum=21.10.2009&Aktenzeichen=2%20Sa%20183/09