Gluetun should be able to do this without any extra tools, check out the qbittorrent example on the gluetun wiki: https://github.com/qdm12/gluetun-wiki/blob/main/setup/advanced/vpn-port-forwarding.md#qbittorrent-example
I do something similar for transmission, though I put it all in a script that I mount into the container so my command is just VPN_PORT_FORWARDING_UP_COMMAND=/bin/sh -c "set-transmission-port {{PORTS}}"


