The Story of My Experiments with Truth—that certainly sounds cool, but what on earth does it mean?
With this title, Gandhi is putting our focus on Truth with a capital T. That's his goal; that's his quest. He wants to find truth, and how does he do that? Like any good (social) scientist: through experiment.
He tries out different approaches—eating meat vs. not eating meat, for instance—and sees what the results are. In light of experience, he's able to say some experiments produced better results than others. We might not agree with his conclusions, but it's his story, after all. (Also: you're really going to argue with Gandhi? Come on, now.)
Finally, he says his life consists of nothing but experiments with truth, so detailing the experiments will make the story take the shape of an autobiography.