Avocados, full fat eggs, full fat cheese, pork sausage rather than turkey, 80/20 ground beef, butter. You do have to be careful with adding TOO much fat at one time, but you should be able to eat enough with a mixed meal in order to get enough fats to up your calories.
Then there's the whole grain and starch thing. And IF you're trying to gain weight the fastest way to do that is to add fat to carbs, BUT it goes to the wrong place. Honestly if it was me, I'd go search out not only what foods you can eat as an RNY, but look at programs designed for males who are trying to add lean muscle. Cuz honestly that's really what you need--even in the absence of being able to weight train right now.
I'd eat saucy gooey casseroles that are very calorie dense (think lasagna, chicken pot pie, stews, chicken and noodles, chicken and rice). You can blend those things and then thin them out with Fairlife milk or broth in order to make them appropriate for Stage 2/3. Mashed potatoes fully loaded with sour cream or full fat Greek yogurt and cheese and green onions. That sort of thing.
But, quite frankly, you really don't need to "gain" just for the sake of gaining. You really just need to not LOSE any more. Here's the "ideal" weight chart that insurance and the med prof uses. It's pretty beyond most of us, cuz it's so low...but look where you are. You're not that far off. If it was me, I'd just chillax about the scale. Eat to your stage, work with your RD to set up a healthy diet that keeps you from losing...then when you can begin a strategic strength training program working out in about 6-10 weeks, then you see an exercise physiologist who is also an RD to put you on a program to gain muscle.
Hope you heal well and get better soon!!!