Hi Rob,
That seems like an odd question to ask. I mean if you had a partial gastrectomy then you really shouldn't be trying to gain weight by drinking, but rather by increasing your calories with solid food. I don't have any of my stomach missing, but I would think a PG would cause your stomach to empty much quicker, which would certainly cause diarrhea if liquids were involved. And liquids dumped quickly into the small bowel of someone with an ileo will wind up just flushing out what's already there and filling your bag. I'd think you'd be better off only drinking like 30 minutes after a meal and never during, and focus on more calorie dense food rather than liquid calories.
Personally, I've found it hard to gain weight, but can do it if I don't drink during meals and leave at least 4 hours between meals for complete (or as complete as it's gonna be) digestion to occur. Now I'm just assuming your pyloric sphincter was removed as part of the gastrectomy. If it was then fats won't get to stay in your stomach as long as needed to be fully broken down for digestion.......and releasing those into our small bowel won't help the digestion situation. So if that's the case you might want to reduce your fat intake as well. I'm thinking a good dietician would go a long way here.
Was any of your Duodenum removed with the part of your stomach....or is it fully intact? If any was removed you need to find out what the nutrients you can no longer absorb are and figure out how to get them....ie, orally or sublingually. Weight gain may be challenging for you, as it is for some of us on here.......so you probably need to get some help with that. But you'll need to understand how your new digestive system has changed........and then optimize for those changes. And I doubt it will be as simple as drinking this drink or that one........if you know what I mean. Your Gastro should be able to point you in the direction of a good dietician that works with IBD patients rather than diabetes patients. I think (hope) what you're calling diarrhea is really just liquid rushing thru your small bowel. Diarrhea would involve irritation of the bowel and is much worse......because then nothing gets absorbed, solid or liquid. Remember that without a colon most of the liquid you drink is going to end up in your bag, as it can't be absorbed. Your small bowel can absorb some........but it's no colon by a long shot. Keep us updated.
;O)