Gaining

So I am having issues gaining by just eating alot. I am already at 150 kilos and it seems just eating alot doesnt do it anymore.

I did try heavy cream mixed with chocolat milk or sugar a bit back and that helped a ton. Gained 2 kilos it 2 weeks, but it left me with bad acid reflux every morning after I drank some. I have some chewing pills for it, but it barely helps.
Are there any tips on how to avoid the acid reflux caused by heavy cream? Some food that balances out whatever inbalance is happening from drinking it?

Heavy cream might not be for you. I know some people do better with it as a cooking ingredient. But you may have to accept that there's too much fat in heavy cream for you to handle.

Before you give up on HC, you should try digestive enzymes. I have to take them myself for unrelated reasons, but here's what I've used. (Take before eating to get the best results)

Zypan by Standard Process. Has several enzymes and seems to be good at handling sugars and carbs.

Essential Enzymes by Source Naturals. Has a different list of active ingredients, and is a good all-around option.

Premier HCL (Betaine Hydrochloride). Basically boosts your stomach acid, so it's good for handling heartburn and dense foods like meat, veggies and fiber. It could be helpful after stuffings when you're taking too long to digest a huge amount of food. Probably bad for acid reflux though.

Ox Bile. This is especially good for digesting fats, so is probably the best option for handling heavy cream.

I have heard Lactaid can be very helpful with digesting dairy, although I'm not sure whether or not it's only useful for people who are lactose intolerant, or if boosting your lactase enzymes would help with digesting large quantities of dairy for someone who isn't.

Regular antacids like Tums can help soothe heartburn, not sure about acid reflux though.

One thing I've taken to help acid reflux specifically is a little bit of baking soda (about 1/2 teaspoon) mixed into water and drink it in sips. It neutralizes acids quickly because it has a base pH. Be careful not to overdo it though. Taking too much or too frequently can put you at risk for some bad side effects. It would be better to get help digesting beforehand rather than soothing it after the fact.

I bought some Ox Bile and it works wonders. Tried it out a few times already, taking one right before drinking heavy cream in the evening and havent had acid reflux or heartburn or how its called in englisch once. I still feel a bit ... weird the next morning. But just eating/drinking something helps that. Not nearly as misserable as with that constant burning felling