How can a crown stay on/in your gum?

It can't. A crown is a durable covering made of porcelain or metal placed over a tooth to strengthen the tooth due to decay, fracture, breakage, etc. There has to be a usable amount of tooth for the crown to be adhered to. The edge or margin can be placed just below the gum line but a crown cannot sit on just gums. Crowns are cemented or bonded to the existing tooth.