I'm fully vegan and have gained almost 15 pounds in less than 3 months. I don't think it's what you're eating that's affecting your gains but how much you're eating. I realized I really have to stuff my face to get enough calories each day. It seems like a pain but luckily for me it was really easy to get used to eating a lot and often.

I have a big bowl of oatmeal every morning with banana, chopped pecans, and a lot of peanut butter. You could also add raisins if it's not sweet enough.

A couple hours later I'll have a snack like toast with pb, hummus, or some tofu scramble I make fairly often (don't be afraid of soy, the estrogen thing is a myth and it's a great source of protein, just don't eat 10 servings a day and you'll be fine).

Lunch is always a big serving of whatever I had for dinner the night before (always make a lot so you have leftovers). Sometimes I get creative with dinner but my staples are:

2-3 cans of beans, 1 can tomatoes, 1 cup corn, onion and garlic plus any combination of cumin, chili powder, paprika, and cayenne. Serve over brown rice and top with half avocado, fresh tomato, cilantro, and lime juice.

Stirfry - dice tofu and fry til brown (it takes oractice to evenly brown), add bag of frozen stir-fry mix, can also add can of beans and fresh spinach/greens. I make a delicious peanut sauce with peanut butter, water, soy sauce, garlic, ginger, lemon juice and agave. Serve over rice or noodles.

Pasta - look up ppk tempeh sausage crumbles, super easy, delicious, and adds protein. Cook the tempeh, add pasta sauce and boom you're done.

Indian lentil curry - I just add lentils and anything else I have, like potatoes/sweet potatoes, any veggies (the more the better), tons of indian spices like curry powder, turmeric, coriander, then like halfway through cooking add a can of full fat coconut milk which makes it super creamy and adds tons of calories. Service with rice and or naan bread. Take a little bit of practice to make it good. Can also use a crock pot for this.

I have other meals I could also add in if you're interested. Hope this helps. Just remember it's definitely possible to gain weight without meat, just have to consistently stuff your face.