Found this thread while looking for the solution as well.
As far as I know slskd doesn't have an API that allows updating the port. If that assumption is correct, even if something existed that would read the gluetun-provided port, the only thing it could do is update the config file and reload slskd, which is disruptive (as it will fail all downloads).
I think the first thing to do would be to open a ticket with slskd to allow updating the port without restarting, ideally through an API endpoint - you could then call that with https://github.com/afiestas/gluetun-sync

Edit: found a feature request on slskd to that effect: slskd/slskd#1011

You can use:

VPN_PORT_FORWARDING_UP_COMMAND=/bin/sh -c 'sed -i "s/^([[:space:]]listen_port:)./\1 $(cat /gluetun/forwarded_port)/" /slskddata/slskd.yml'

You have to mount the volume or at least the slskd.yml file to the gluetun container to have it modify the file. The file updates runtime automagically.