Gaining

I was a real slow gainer too but I kept up with eating lager portions most every day and eating a bowl of ice cream right before going to bed.
I did the WG shakes every few days too.
I gained 5 or 10 lbs here and there and after a few years my metabolism slowed way down and the gains were coming in 10 and 20 lb increments and the weight was staying on.
In a 5 years span I went from 115 to 220 lbs which isn't all that astounding.
Some people gain really easily some of us don't.
I will say once you start gaining its really hard to not put on a few more lbs without really thinking about it.

Whatever amount you are comfy with.
Weight gain shakes used to hurt my stomach when I first started drinking them.
I found that in the beginning stuffing myself would give me indigestion too.
After I had gained some weight maybe 40 lbs I was able to stuff myself properly and I loved the full feeling.
I deal with compulsive over eating disorder too and from time to time when it flares up I will stuff myself silly.
I am not an active gainer like I was years ago.
I am still a fatty because I like being fat.

I'm really new to actually trying to gain. Most of my life I have found it hard to gain and retain weight, except for some times with medical conditions, medicine, or pregnancy. I run with a high speed metabolism, which may have slowed down some.

Already, with 3 weeks or so of trying, I am noticing some good soft fat in various places with heavy cream. There are many ways to drink it without much or any sweetener. I recommend starting small and going up, like 4 oz, work your way up to 8, then 12. Depending on what else you eat, your body may not be able to digest much more daily, despite the larger amounts some here and elsewhere mention they consume without indigestion. It may be possible to work up to more over time as the body realizes what is incoming.

I use strong (high dose) digestive enzymes (you want a lot of lipase to digest fats) to help with the digestion. As magnesium and selenium are critical ingredients for many enzymes, these are good to take, for your body's own production of digestive and other enzymes, like to help with food coma. And calcium is needed to let the heavy cream work its best because it causes adiponectin production in the body, and a critical ingredient for that is calcium. Maybe there is enough in heavy cream, but I take some added calcium or eat leafy green veg which have a lot of calcium. So far, I have not consistently been eating that much more, but the heavy cream is doing its work. You might give that a try. It is better to do it for a while, I've heard 3 or 4 weeks, then take a week off, to keep your hormones guessing so they won't adjust, I think. Good luck!

You also may want to try probiotics geared toward the digestive/GI system. Microbiomes that are imbalanced can contribute to the indigestion.