Why does my dog keep gagging?

best bet is that its a hairball, unless the gagging gets out of hand, they'll be able to take care of it themselves

Gagging is a symptom that is similar to choking and coughing. While your pet may not actually be choking on an object or a bone, he may be suffering from a condition known as reverse sneezing. When your pet sneezes, he releases air from his nostrils, but if he’s suffering from reverse sneezing he will rapidly take in air through his nose and make loud coughing, hacking or snorting sounds. Although reverse sneezing is just temporary, pet owners are often alarmed when it happens. If the episodes recur intermittently throughout the week, you should get the pet examined by the vet to rule out underlying health conditions.

I recommend your dog be checked out by a vet to rule out kennel cough, or something worse. How long do the gagging episodes last and how often do they happen? What is your dog doing when a gagging episode starts? Any discharge from his nose or eyes?