Earlier quoted context omitted.
What actually is a 4to6 gateway? For example, how would an IPv4 client connect to a 128-bit address like 2606:4700:20::6819:6c0d?
You contract with a tunnel broker (most, if not all, are free) to give you an address. If you have a lan then you can set up radvd (or similar) to distribute addresses. You may need to replace your router though. Your ipv6 packets head out over a 4to6 tunnel to your broker, and then out to the wider world.
All the complexity is about providing this to the end users. Depending on your tech stack, you may have no issues, or you may need to replace just about everything (NASes, accounting/billing system, etc - whatever pieces were built with IPv4-only in mind)
Source: I worked for an ISP a while ago. We got our servers IPv6 connectivity, in, like, a day. We haven't solved IPv6 end-user connectivity (but it really wasn't a priority then), although was we had a few experimental connections - with lots of duct tape and random end-user issues whenever something was misconfigured (e.g. at the time not every site's AAAA record was actually responding, despite the existence).