I think any individual fish has the capability of doing so. Especially on something with high caloric content like pellets. Also, pellets are made from dried meal, so they need not be fed in the same bulk as moist/raw foods, which are mostly water. Similar to dry dog or cat food vs wet or raw. At the least it will get fat and you will limit its lifespan and cause health problems. So you need to make sure to limit its food supply.
Looking at the nutrition info for your pellets they're 10%+ fat which is a bit high. I can't seem to find any calorie info.
Pellets are a more complete meal than meats and live food but you cannot just feed as much as they want, anymore than you can let the average labrador retriever freefeed at the bag.
My original one doesn't eat a huge amount before he's full, so he limits his own food intake, and we feed him fairly low fat foods like tilapia filet. But some won't do that. I've seen some VERY obese gars.
As far as lethargy - They mainly float, but he should be active part of the day, and usually float slowly across the tank rather than one spot. You say he's not getting picked on, but gars usually aren't safe with cichlids. If he's sitting in one spot maybe it's to avoid being picked on.