A network tunnel works in a very different way from a translation gateway. For one thing, it is a two ended scheme. One end embeds packets of one protocol (e.g. IPv6), including all headers, as the data part of a packet of another protocol (e.g. IPv4). This allows the resulting packet-within-a-packet to travel over a network of the other protocol (e.g. IPv4) to the other end of the tunnel, where the header of the second protocol (e.g. IPv4) is removed, leaving the original packet in the first protocol (e.g. IPv6).
This allows sending IPv6 packets in both directions over an IPv4 network. The intervening network (e.g. your existing IPv4 service provider) doesn’t even realize the hidden IPv6 packets are there. When sending IPv6 packets through IPv4 this is called “6in4 tunneling”. When sending IPv4 packets through IPv6, this is called “4in6 tunneling”.