In order to gain muscle, supplying your body with enough calories and protein throughout your day is crucial, along with regular resistance training.
While following a vegan diet, protein is going to be a nutrient that is difficult to consume enough of, since all animal products are excluded. However, it's still possible to meet your daily protein needs. Including plant protein sources (e.g. soy, nuts) as well as consuming a plant based protein supplement can help you get to your protein goal.
Building muscle while following a vegan diet can be more challenging, but is certainly possible. If you'd like to discuss further, reach out to your registered dietitian by going to the "Registered Dietitian" tab of your Gainful account.