First I'd politely ask the loud gum smacker if they'd mind not smacking their gum because it makes it difficult for you to hear and/or concentrate ...

If that doesn't work, then I'd LOUDLY state in an assertive manner to the instructor ... Excuse me Mr. / Ms. (Name), Would you mind repeating that? ... I can't hear you over the loud mouth gum smacker back here ... Thank you ...

Good Luck & Best Wishes ... !!!