in kakoune, this is `<a-i>p<a-j>|reflow<ret>`. which is obviously not as simple as `gq`, but you can rebind this, add things like auto-reflow as you type, etc., rather trivially, and rebind them (and a lot of these types of tools come preloaded as bindable commands). it's some work to set up this type of thing (and it makes perfect sense to use e.g. vim to not need to do that), but i like the composable design, where i can write some configuration to hook a shell command to a bind, rather than fork and PR changes (e.g. a c++ reflow function), and the config files can be written and PRd into the default distribution as well, so the user experience would be equivalent.
apologies, i probably should've written `fmt` there (which does the same thing). `reflow` is a `fmt` clone i wrote, without the knowledge `fmt` existed --- that was a bit embarrassing for a friend to eventually point out, that my idea was already in coreutils :D i use `fmt` now because i figure it's far better tested (i think it's posix), but i still think of the action as a "reflow", and that's also the term helix uses.
'|' is the command to pipe a selection through a shell command and replace it with the output, so I guess it's just a regular command on their system. You could use any other reflow or reformatting program you had handy. Kakoune in particular is really serious about delegating everything possible to the system.
Honestly, it might have just been a placeholder name for an arbitrary formatter. I vaguely remember old-school unix had a command for that, but none of them seem to be named exactly "reflow".
It's a nice project though, I just use that feature constantly