Is there a GreenVet product that will treat a preening gland infection?

You can use either Apagel Spray or Apagel (cream). Apagel Spray is a bit more concentrated, but as it is liquid a bit more difficult to apply. Apagel (cream) is denser and therefore easier to apply but may require a few extra applications as it is less concentrated. Apply either product around the preening gland until the infection clears up.