Autobiography
Yup, this is definitely an autobiography. In fact, sometimes The Story of My Experiments with Truth is considered a subtitle and An Autobiography the actual title. That makes identifying the genre pretty straightforward, right?
Now, Gandhi sees the book—as he explains in the introduction—as an account of his experiments with truth (sort of like a lab report). But, since his "life consists of nothing but those experiments, it is true that the story will take the shape of an autobiography" (Introduction.3).
That means we find out about his life, but it's definitely not written in the talky vein of, say, Angela's Ashes or the lyrical style of Out of Africa. Instead, everything in this book is geared toward examining what Gandhi's experiments in politics, diet, and other fields have to say about truth as he sees it.