I managed to gain 4 pounds from my lowest whilst still on antibiotics over about 6 months, but felt like I was constantly eating or thinking about it!
I did eat ice cream with hot fudge and caramel before bed almost every night during that time. I also put Orgain a double scoop protein'n'greens in my smoothies, and tried to drink about 12 oz a day (keeps for 3 days in refrigerator.)
My shake recipe - 1 c full-fat Greek yogurt, 2 c fruit (mixed berries, mangoes, bananas, oranges - fresh or frozen) one carrot, one large avocado (or frozen equivalent), two large scoops protein powder, blended with milk or ice. Sometimes a little peanut or sunflower butter. (I think 6 ounces was 120-150 calories depending on what was in it.)
Sue