Is there a mouthwash or anything that can stop gingivitis?

Absolutely: Start with a visit to your dentist to have your teeth professionally cleaned and education on good home care. Brushing 2x/day and flossing and using a antimicrobial mouth rinse will keep your gums healthy

Not stop, but help: Mouthwash alone will not stop gingivitis but it can speed healing and help prevent reoccur acne when combined with good oral hygiene. Listarine and Crest Pro Health are two good over the counter ones. For more severe cashes your dentist can prescribe Peridex (chlorhexidine gluconate). Have a good cleaning done by your dentist to give you a fresh start.

Definetely, yes: By definition gingivitis is inflammatory, but a non-destructive, curable disease. After the first step, removing the source of the infection by your dentist, the home treatment is the key. Brush and floss properly 2 times a day and see your dentist every 6 months for prophylaxis - the best cure for gingivitis.