What is the best treatment for gum pain after wearing night guard?

Adjust the guard. You need to go back to your dentist to have the night guard adjusted so that it isn't putting pressure on your gums.
Adjust night gard. First find out why you have a pain. There are several reasons why you have pain. Look for source of pain. Either teeth, gums or nightguard or your own habit or cause of grinding.
Mouthrinse. Rinse your mouth with salt water or your favorite mouthrinse. Then, take your nightguard to your dentist to trim it so it doesn't rub against your gums...If this continues it can cause other problems like gum recession, sensitive teeth...