In the Farewell at the end of the book, Gandhi explains why he's stopping his autobiography in 1921…even though the book was published in 1927, which means that he wrote the bulk of it in the mid-1920s.
His life since 1921 had been very public, so he figures people already know a lot about it—and plus, in order to explain his current experiments in politics, he'd have to discuss people he works with. (It's never a good idea to share gossip about living political leaders, especially if you're also a living political leader.) So, that's why Gandhi signs off at 1921.
It's not Gandhi's style to just say, "And then it was 1921. Happy New Year. Peace." He adds some big-picture statements in conclusion. We hear once again that God is truth; we hear that the means to achieving truth is ahimsa, or non-violence; we hear that self-purification is necessary to identify with all life and thereby observe ahimsa and thereby find truth.
He also says that devotion to truth draws you into all areas of life, and therefore, religion and politics do mix—religion being truth and politics being an area of life—even though people often say they don't.
Finally, and perhaps most touching, is Gandhi's closing admission that he's not perfect. He hasn't achieved "perfect purity," and he hasn't become "passion-free in thought, speech and action" (Farewell.7). He says he still has "a difficult path to traverse" (Farewell.7)—there's always room for improvement, in other words.
Maybe Gandhi is being a bit too hard on himself, but then again, perhaps that's what gave him so much strength in the first place. He closes with a prayer, asking the reader to join him in wishing for "the God of Truth" (Farewell.8) to grant him ahimsa in mind, word, and deed.