Frenulum ripped, can it get gangrene, what to do?

Several: Frenulum of tongue? Frenulum of penis. Either should heal with cleaning and a week or so, and some topical antibiotic jelly if penis. Both are vascular tissue and gangrene is unlikely.

Very unlikely: if you don't have any underlying condition that suppress you immune system. You can get other infections, however, so try to keep it clean and use antiseptic mouthwash for few days until it heals.