How can I make my dog gain more weight?!?

I'm not a dog trainer or vet or any such thing, but, having had numerous dogs over the years, including a German Shepherd, my wife and I found that our dogs eat noticeably less during the hot summer months, just as a regular course of events. But, wanting our dogs to keep to a healthy diet, my wife started staying with our dogs till they had finished their bowls of food, or at least as much as she could coax them to do. That way she could encourage them to eat all their food, plus, it gave them the personal attention they always like. She was also able to make sure that all the dogs ate all THEIR OWN food and didn't rob each others' bowls, as sometimes our male dogs would to the female dogs. She might also include some food / leftovers we have here in the house for a special treat, like spicing up their life a bit with variety. By "spicing", I don't mean actually giving them spicy foods which may not be good for them, but she might have some leftover turkey that she'd cut up extra special for them and mix in with their regular foods, and they've always loved that. I can also recommend a treat sold at stores that ALL our dogs just go nuts for, and that's "Waggin' Train Wholesome Chicken Jerky Tenders". They're a bit more expensive but, if you want your dogs to eat more, this will DEFINITELY do the trick. You can break up one and mix in their food, or you can give them as a treat. We've found if you break them up and mix in their food, they'll eat their regular food much better. WARNING: The first one you give them as a treat they'll gently take from your hands. Thereafter, MAKE SURE you hold the smallest end, at the very tip, and be ready to let go ASAP!! Mine won't mind taking your fingers with the treat if you don't get your fingers out of the way!! NO JOKE!! And do break them in half at least since they love them so much that they forget they do need to chew them before they swallow them, and can sometimes begin choking on a whole one, so, break them in half or thirds, and if you put them in the palm of your hand and let your dogs take them that way, you'll be more likely to keep all your appendages. It may be best to break them up and put them in their food so they'll eat more of their regular food as well.