As evidenced by the athletes and science featured in The Game Changers, gaining muscle and strength on a plant-based diet shouldn’t be an issue, since the plant kingdom is full of protein-rich foods, which means all that people who are looking to gain size and strength on a plant-based diet need to do is educate themselves on which foods those are and include lots of them in their daily eating schedule.
More on that, at What About Protein and Gaining Muscle and Strength and The Plant-Based Advantage.