Could you pls help me for which one of the following is correct ?
This is the gentle reminder Or This is a gentle reminder
I think both sentences are correct
firstly , you have to concentrate on the text which mentioned this sentence
The first time you speak of something use "a or an", the next time you repeat that object use "the".
It depends on what the speaker wants to express and to what he/she is referring. If the speaker is referring to a "reminder" that someone inquired about, then the speaker could say, "This is the gentle reminder". But if the speaker is simply referring to some suggestion or recommendation that was made previously and wants to "remind" someone or everyone about it, the speaker could say "This is a gentle reminder".
Both can be used I think but see situation first.