'Gentle reminder' by Dominik Gajarský at Vitrína Deniska, Olomouc

Hi, I remember receiving emails titled "Gentle Reminder" from University administrative staff. But I am not sure if this is a friendly phrase. You may be wondering what I mean by friendly. Let me give you an example of what I have in mind: Please take off your shoes before entering the house. --> friendly Kindly take off your shoes before entering the house. --> not friendly [Well, at least that's what I have been told.] The reason I am asking about the phrase "Gentle reminder" is that I want to write a short note reminding my referee (person who writes reference letters for me) that the letter is due in a few days. Would "Gentle Reminder" be a friendly phrase to use in this context?: Hi, XXX, Just a gentle reminder that the reference letter for the position of [name of position] is due on XXX. 
Thanks very much. 
Thanks!