Is it true that the best diet to eat for gaining muscle is one of meat?

No, that's outdated: That is an outdated thought. An appropriate diet to build muscle is one that provides enough calorie and protein to support your body's muscle building efforts while getting enough vitamins, minerals, fiber, etc that are just as essential to good health. Focusing on meat will almost certainly provide excess protein & calorie with too much fat and not enough vit/minerals/fiber.