As the leader of India's independence movement, Mahatma Gandhi gained lasting international fame for his non-violent activism. He is an enduring historical icon, and a well-made costume of him will be highly recognisable. Although Gandhi and his style changed dramatically over the course of his life, he's most remembered as the old man of his latter years. As with all costumes, the key is to identify the essential elements of Gandhi's appearance and replicate them. Your replicas don't need to be perfect.
Shave your head or find a "bald" wig. Alternatively, use a skin-coloured swimming cap. Gandhi shaved his head, and he would be unrecognisable with a modern haircut.
Purchase cheap sunglasses with round lenses and punch them out. Alternatively, create your own round glasses with wire. Gandhi was rarely seen without his spectacles, and they're as iconic as his shaved head.
Wear a pair of loose white capri pants or trousers. They should not go past the ankles.
Put on a white tunic and then drape a white sheet around your shoulders the same way you would wear a sash.
Slip into a pair of sandals.
Purchase or find a straight wooden walking stick, approximately shoulder height. Peel off any bark on it. Use the stick as a walking cane. You're now Gandhi!