Is it safe to use hydrogen peroxide for gingivitis treatment?

Not really: It's a good treatment for acute infections. Long term, there is risk of oral lesions, carcinogenesis and bleaching soft tissues which could mask the redness of inflammation. Proper plaque control, brushing, flossing, listerine should be combine with professional treatment.

Yes but unecessary: Hydrogen peroxide treatment was made popular by paul keyes in the 1980's in treatment of gum disease. It is only effective in gingivitis but not periodontitis, as proven by the american academy of periodontology. However, with good oral hygiene of brushing and flossing and a regular periodic cleaning at your dentist/hygienist, gingivitis should be easily controlled.