Well, you've already put up some obstacles by telling us off the bat that you can't eat much and that you can't go to the gym. So let's get a few things straight. Yogi is absolutely right - but to tailor advice further....
1) is it just HARD to go to the gym, or impossible?
2) is a home gym an option? It doesn't have to be a full setup - there are many ways to "work out" - but if you CANNOT go to a gym, let's figure out what you have to work with.
(For what it's worth: if it's just HARD to go to the gym but you CAN go even 2 days a week, there are lifting templates written for that)
Thingies you can look into if a full home gym (power cage, squat rack, bench) is not an option:
- pull-up bar
- wheelbarrows filled with rocks/dirt/sand
I am, of course, partial to the kettlebell and would tell most anyone that two good KB's will get you much farther than no exercise equipment at all. If the choice is "no training" that's certainly better than nothing. It depends how much mass you really expect to add, I guess, but I'll assume you're not expecting to turn into Hulk Hogan in 6 weeks.
Point is, if going to the gym is not an option for you, fucking figure out a way to do some work at home. Maybe that means loading your wheelbarrow with a bunch of rocks and pushing it around, then lifting the stones over your head.
3) let's talk about reflux and not eating very much. Details please. What you can eat, how much, etc. is this just "I can't eat before I go to bed" (eating at least 2 hours before bed has helped my GF wean herself off reflux meds). Is there a way to cram in lots of calories at certain times during the day? Are certain foods a trigger? Have you done any kind of elimination diet to see?
Potentially unpopular opinion alert: going mostly Paleo (with judicious addition of clean carbs when needed) has done wonders for me in many ways, and there are many ANECDOTAL reports of folks that had things like reflux "clear up" when they stopped eating junk foods.
Also: is it just a problem with eating large "volumes" of food? Because there are ways around that. Calorie dense foods can be snuck into a lot of places. Heavy cream in your coffee or protein shake. Extra butter on your vegetables or baked potato. Olive oil on a salad. Just plain eating a spoonful of coconut oil. Stuff like that.
If you're drinking skim milk and eating lowfat cereals and fat free yogurt, of course you'll have to work harder to get calories in.