I haven't gained weight but have just as hard of a time losing it, which has been disappointing because I was kind of hoping poor sleep was the main reason for my snail's pace metabolism. I was inspired by many people talking about all this weight loss and other health improvements simply from starting CPAP therapy, but mostly I've found all the same health problems are still there, I just feel a bit more rested during the day.
I suspect the exponential increase in the difficulty of losing weight is related mostly to my age now. I used to go low-carb or cut calories to 1200 and lose 20 lbs in a couple of weeks - now it's more like 7-8 in a month, if I'm lucky. :-/
I too find it hard to lose weight. As someone here posted a few months ago here on the same topic, after cpap therapy initiated - food looks better, smells better, and tastes better.