To put together a mass-gaining meal plan based on plant foods the objectives are no different than they are on any diet. To build muscle you will need a calorie surplus (eating more calories than you burn metabolically and through exercise) from healthy whole food sources like fruits, vegetables, whole grains, and beans/nuts, and getting plenty of protein. You will also need to create the demand for more muscle through hard training and adequate recovery. Lastly, you will need to these things consistently, day in and day out, for long enough that a change can take place. Great physiques take time and commitment.
If you consume enough quality calories depending on your activity level, you’ll be fine. Calories from protein are often used in muscle repair, which is what causes muscle growth. If you don’t eat enough calories overall though, your body is much more likely to use your protein sources as fuel instead of for growth.
If you are looking to build Muscle Mass and are following an intense weight training program it’s a good idea to make sure you consume more of the protein dense foods like beans, nuts, seeds, and whole grains (and steer clear of processed foods like fake meat products). These are also the most calorie dense plant foods, which will make it easier to create that calorie surplus. As for supplementing protein, there are several great protein options based on whole plant foods that will make a great post-workout or meal replacement shake.