I have also gained weight since diagnosis. It's about a pound per month, which I think is mostly due to sedentary lifestyle and lack of exercise, though I do admit my diet, which was mostly vegetarian, has become much more liberal.
I mean, if you have ALS, but are lucky enough to still be able chew, eat and swallow, who wants to just live on salads?
I used to work out regularly, but now I only walk distances of less than 100 feet. Anything longer requires my scooter, so not much caloric burn there. And, of course, I sleep a lot and nap also. Average 9 hours per night on Trilogy, though sometimes do 14. Not much caloric burn there either.