Hello. You did not show the letter from Ghandi to which the question refers, which makes it impossible for your question to be answered accurately. However, to help you, I will show you how to recognize a credible letter and a non-credible letter.
A credible letter is one that presents statements that are confirmed with real supporting details, that present evidence that makes the reader sure that everything written in the letter is consistent with reality and can be proven. If the letter does not have these elements, it is not credible and is often not relevant.