You should look up Dr. Michael Greger. He runs a website and youtube channel called nutritionfacts.org that aggregates information on dietary science, focusing on the health benefits of eating whole-food plant-based (WFPB) (which specifically does not include cooking oils). He has also put out two books (with corresponding cookbooks), one titled “How Not To Die” and the other “How Not To Diet”. I have only read the forward to the latter’s cookbook, which condenses the information into just a few pages. I have also used some of the recipes which I thought were some of the tastiest recipes I’ve used out of a book. However, I will say that recipes feel a little redundant for WFPB since every dish is essentially a variation on legumes + grains + leafy vegetables + starchy vegetables (+ broth for a soup/stew/chili).

Regarding you question about calories and protein:

• In general, if you eat a balanced WFPB diet of equal parts of the above, you will not have a protein deficit; however, I don’t know if this applies to trying to bulk up at the gym where you would need more protein to build muscles. Legumes have some of the highest protein per mass of any WFPB food (as do some seeds/nuts), but the highest out of every food is seitan which if I remember correctly is 90% or above protein by mass since it is essentially the gluten sifted out of regular flour. (Seitan is not quite WFPB but it should still be very healthy unless you are gluten intolerant or you eat so much that you get protein toxicity.)
• In general, WFPB foods are calorically dilute (with either fiber or water) compared to either junk food or animal-based foods. If you are switching to WFPB from something else, you literally have to eat more than you are used to or you will have a calorie deficit. If your food intake is truly WFPB with no processed oils you should not be able to gain weight above a certain level because of the amount of fiber in your diet and the physical volume constraints of your digestive system compared to the low calorie-density of WFPB foods. (Always eating WFPB is extremely difficult because you can basically never eat out at a restaurant. I also don’t exactly follow WFPB since I eat bread/tortillas and eat out sometimes, but I use it as a starting point for when I am cooking at home.)
• Personally I find whole grains to give me a stronger sense of satiety and energy than any other food type, since they are high in carbs and fiber but low in water mass. They also have some protein. If you are feeling hungry/low energy/like you are not intaking enough calories, I’d probably recommend increasing your intake of grains. In comparison, making a soup/stew/chili dish will give you a sense of fullness but you will consume significantly fewer calories because the physical volume of you stomach will be filled with more nutritionally empty water—so great for weight loss but not for gaining or maintaining weight. Similarly, you can lose weight on a diet of white potatoes (no added oils) since they are so satiating that you will end up with a calorie deficit.
• The most calorie dense WFPB foods would be anything with oil, which would be nuts, seeds, peanut-butter (I think it is still WFPB because the processing is very light and does not discard anything), and avocados. If you are worried about maintaining or gaining weight with a healthy WFPB diet I’d recommend increasing your consumption of these foods.
• WFPB does not include cooking oil for essentially two reasons. First, oils are highly calorie dense when they are not in a whole food bound up with fibers and other nutrients, it is easy to gain weight. Second, there are some negative health effects (especially for the circulatory system) of eating too much processed oil (even olive oil), however oils that are solid at room temperature are worse. If you’re extremely worried about your weight being too low, you could include a small amount of olive/canola oil in your diet to gain some weight without too much risk.

As a disclaimer, you should not take this as medical advice (I am a non-professional operating from memory). There are also different schools of thought in nutrition so different health professionals may give different answers based on when they learned nutrition and how in-depth their studies were. You should look into the people on which I am basing my suggestions and see what you think yourself. In addition to Dr. Michael Greger, some other proponents of WFPB (or something similar) I can think of off the top of my head are Dr. Neal Barnard, Dr. Caldwell Esselstyn, and Dr. Dean Ornish.