They are right, corn is not good in dog food and even tho many companies will say that the protein from corn gluten meal is fine for dogs, it isn't as digestible and can lead to allergies.
When you look at the ingredients on a bag of dog food (such as Drs Foster and Smith brand# you will want to especially look at the first 5: you want to see meat or meat meal in at LEAST 2 positions in the first 5 ingredients. Look that the grains included #if you choose a food that contains grain, which you might want to avoid for your puppy# are not corn, wheat or soy - or any of those glutens - but rice, oatmeal, barley are better.
Any veggies or additional additives such as herbs or supplements are in such small amounts to be almost useless so the fewer, higher quality ingredients, the better.
DFS dog is a very decent food which i feed my dogs #both adult and senior#; if you choose to look for a grain free food, those are options as well and many articles online discuss pros and cons of grain free diets. Find someone you trust #a holistic vet or trainer) to help you choose the diet right for your dog depending on activity, any health issues, past feeding, etc.