I have a dog I recently adopted that teeth were neglected. The previous owner mentioned that the dog didn't eat much. I figured it was because it was old and didn't need as much. That wasn't the case. Turns out Dirtnap had horribly bad breath, missing teeth, and mouth full of decay. He wasn't eating because it hurt him so bad!!!
What I did is I added water to his dry dog food. It softens it some. THROW OUT the uneaten food after a few hours. The food can ferment and develop maggots quickly with some foods. I kept Dirtnap on this water down food for a few weeks. Gradually lessening the water content. You want to keep the food as hard as possible. This helps build up the teeth's strength. It also helps in cleaning. Softening the food will just ensure the cat is able to eat and get nutrients while it's gums/teeth heal.
I have found a great tooth care item!!! They are DENTAL WIPES!!! The wipes are MUCH gentler on their teeth than brushes. Although you do have to put your fingers in their mouths! This is actually easier to do if your bonded. To start out this method it is recommended to have the animal facing AWAY from you when doing this method. They are easier to control and to introduce the wipe/finger to it's mouth withOUT too much biting It will take a few attempts but I can now do it to my cat and 3 of my 4 dogs. (New adoptee in training)
Dirtnap's breath is slowly improving and he is NOW able to eat with the rest of the dogs. I avoid can foods because they are so fattening and don't provide good dental health. If this continues to be an issue try feeding tender Vittles. My cat LOVES it and it is soft but not goey.
Good luck!!! The wipes you can get at a pet store for under $5!