Gehenna is an Old Testament word for the place where God executes eternal judgment. This word, as well as the quotation from Isaiah 66:24, shows that Jesus speaks to children of God as members of the covenant, and warns them (believing disciples) against the punishment of this covenant.1
43 And if your hand causes you to sin, cut it off. It is better for you to enter life crippled than with two hands to go to hell,to the unquenchable fire.