It is very hard to prove that somebody did not say or write something; absolute certainty is not possible. But one can be very confident when the citations don't work out, reasonable alternatives to the citations don't work out, when it can be shown the person did say something rather different in the same time frame when the alleged statement was made, or best when the "quote" can be traced to its original source.
None of these following were said by Mahatma Gandhi, with high confidence.
This first one was current around 1990, during the height of the dispute over the Ramajanmabhoomi site in Ayodhya where Babur or someone had built a mosque.
"The mosques built over the destroyed temples are a sign of slavery and the Muslims should hand over the same to Hindu Society." Mohandas K. Gandhi, 'Navajivan' July 17, 1937.
I had done an extensive search for this back then (1990-92) and pretty much showed that Mahatma Gandhi didn't say or write that. The details are buried somewhere in my boxes of notes.
""Hitler is not a bad man" - Mahatma Gandhi never said or wrote that either,
see here for details of why I think so.
"Mahatma Gandhi said: “The press is called the Fourth Estate. It is definitely a power, but, to misuse that power is criminal." - this was actually used by PM Narendra Modi in a tweet.
It is supposedly from a prayer speech, April 12, 1947. But it is really from a newspaper report of that prayer speech. The transcript of the speech says no such thing. The original quote is: "In the English language the Press is called the Fourth Estate. The Press can help or harm the country in so many ways. If the newspapers do not maintain a healthy attitude, what purpose would be served by India becoming free?" Gandhi had said earlier in that speech ".....it is a trick of their trade to create panic among the people and thus increase their sales. It is a very wicked thing to indulge in order to fill this wretched tummy. ...If for earning their livelihood these people fill he pages of their newspapers and thereby harm the interests of India,
then, they must give up journalism and find some other occupation."
However much of Gandhiana takes this quote to be accurate.