One recommendation for patients with gastritis is to make sure to get plenty of the live cultures of bacteria found in yogurt (as long as the yogurt says that it contains active cultures). These are helpful for reducing H. pylori in the stomach (they are also good for counterbalancing yeast and a variety of other helpful things). According to one popular acid/alkaline food chart, yogurt's properties aren't clear in the general information out there, so it isn't terribly strong.
For simply providing calories, fats like butter are going to be fairly neutral. You also might want to consider oils from low acid nuts like those you mentioned. These things can amp up calories in the diet. Unfortunately what they don't do is provide protein, which is important for the human diet. You can get protein from tofu and other soy products (slightly alkaline). Tempeh and wheat protein powder (which you may be able to find as textured vegetable protein, a staple for backpackers) also look to be lower in acid. Vegetarian blogs such as Michael's can provide you with interesting recipes for tofu, tempeh, and seitan (a wheat gluten product that is similar to wheat protein powder), although you will have to watch out for what the ingredients are.
For more carbohydrates try quinoa. It's rich in protein and fiber and helps to promote a balanced diet.