Teeth should bite against the teeth in the opposing jaw. If the jaws are not aligned, or if there is a mis-match in jaw size, or if the teeth are displaced, you could end up with your teeth biting on the gums. This should be corrected with orthodontic treatment, to prevent injury to the gums. Since the gums support the teeth, extensive loss of gum would result in a weakened, shaky tooth.
See also: Open bite