Would people's teeth with gingivitis go back to normal before they had gingivitis, when treated?

Usually. Usually. If the gingivitis was successfully treated before it progressed to periodontal disease.
Severity. Gingivitis, swelling of the gums, is reversible with professional cleaning and excellent oral hygiene. If more severe periodontitis, irreversible tissue and bone damage occurs. See your dentist or, if recommended, a periodontist.
Vague. What do you consider "back to normal." do you mean loose teeth, appearance, biting? Gingivitis is treatable but must be continually worked upon by proper home care and consistent visits to the dental office.