Could someone possibly gain weight by eating a lot of calories but from healthy foods?

Of course: Control of portion size is also an essential part of weight loss.

Yes. Foods to help: With healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocadoes, granola, trail mix, dried fruits, extra virgin olive oil, canola oil, protein shakes, protein bars, lean meats, poultry, fish, smoothies (with added silken tofu or powdered milk), greek yogurt (full –fat), pasta/ bagels ; use of juice rather than soda. Some people add whey proteins.