How can a vegetarian gain weight in a healthy way?

I have been struggling to gain weight as a vegetarian. I don't eat mammals or birds but eat invertebrates and have recently had to start eating fish again because of my attempts to gain weight. I want to gain muscle to be stronger and have more stamina. I have a light intolerance to gluten and dairy so some vegan dishes are greatly appreciated too! If anyone has some suggested work outs that wouldn't be to taxing that would be great too!