Peptamen, Ensure, and Boost are all enteral nutrition formulas. Ensure and Boost have whole protein in them, and Peptamen has protein broken down into peptides, which is less likely to cause allergic reactions for people with food allergies. Of the three, Peptamen has been studied most to show that it can induce remission in people with Crohn's, but all three are likely to work just fine. They don't taste much different from each other, although some people like one more than another. If you get the Peptamen, make sure you get the vanilla flavor (along with any other flavor packets that you want), and not the unflavored type which is made for tube feeding and does taste terrible.
These formulas can put you into full remission (no pain, no symptoms, normal blood work) if you are willing to drink only formula and not eat regular food for a few weeks. Not everyone responds, but many do. You will usually notice significant improvement within 10 days if they're going to work for you. As far as weight gain, it depends on how much of the formula you drink. If you are normal weight when you start, and you normally eat 1800 calories per day to maintain your weight, then you should just drink 1800 calories per day of the formula and you will neither gain nor lose weight. If you are underweight and need to gain weight, you will need to drink more of the formula, and if you have very active disease causing weight loss, you may need to drink more calories than usual until you go into remission.