Only get gum pain when opening mouth. why?

Not sure: I would need more information to answer you properly. An educated guess is possible referred pain from your temporal mandibular joint (tmj).

Air sensitivity: Your teeth may have localized gum recession exposing the root surface which may be sensitive when air hitting the root surface during your mouth opening. Treatment of choice for teeth sensitivity is Fluoride tray, which allows adequate Fluoride concentration on the surface for ion exchange (adsorption). Consult your dentist for other probable cause such as cavity.