My son only started gumming at 13 mths. We taught him to gum by giving him Gerber puffs and getting him to bite it. We didn't give him the whole thing though it's so small. We made him bite into 2.
Soon after, he start gumming bread then it became french toasts and pancakes.
Take it a step at a time. Some kids just need more exposure. My son still can't drink from a straw. Hahaha.