Miscarriage at 8wks, discovered at 12wks, had medical management, when will my hcg drop to 0, my boobs have gone down + my gums no longer bleed?

Very soon: HCG takes about 6 weeks to drop after complete miscarriage. Your signs suggest that it is going to be soon. Maybe 2 weeks more

Bleeding gums: Although bleeding gums may be in part hormonal, the most common cause is gum disease. Please see your Dentist, especially if it has been more than 6 months since your last professional cleaning and examination.