Could mouth breathing cause gingivitis?

Absolutely: Mouth breathing can most certainly cause gingivitis! to minimize the gingivitis, first you must have excellent oral hygiene including daily flossing and brushing. Also it might be prudent to see if you have a sleep disorder such as sleep apnea or even plain old snoring both of which could cause the mouth breathing and thus the gingivitis. Best to start simply with meticulous oral hygiene.

Of course: Many factors linked to an elevated risk of gingivitis and the mouth breathing is one of them.