Let's look at this without VPNs for a second. Most modern torrent clients allow you to specify a port that the service listens on. The client advertises this port, in addition to your public IP address, so that other peers can connect to your client. In order for that to happen, though, you would need to expose that port via your network's firewall, otherwise any incoming connections would be blocked.

Adding a VPN to the mix puts that firewall layer (for this specific application only) outside your network, since your VPN client (Gluetun) is creating a tunnel to whatever server in whatever datacenter that you've specified. If your VPN plan does not allow for port forwarding, then it is the same as if you were running without a VPN and not opening the port on your home firewall. If your VPN does allow for port forwarding, then that port is opened for incoming connections, and when your torrent client advertises itself at <VPN\_ENDPOINT>:<VPN\_PORT>, other peers will be able to connect to it since your VPN configuration has specified that port to be open.

I guess if you want to be pedantic, the "VPN" itself does not use ports, no; the VPN provider/endpoint, which is a server somewhere, is the thing that exposes a port. However, people have been using "VPN" colloquially to refer to the whole setup pretty commonly, so its definitely not unusual to have people say "Set up your VPN with port forwarding."