They want you to install their client, but it's wireguard under the hood, so you can make it work with glutun.
You need to generate a wireguard config file with this tool: https://github.com/ViRb3/wgcf
There is a bug with gluten which make it not accept nameservers for a custom wireguard config: https://github.com/qdm12/gluetun/issues/2042
So you will need to resolve the IP yourself.
Personally, I set it up through TrueNAS, and I didn't managed to make it work with a file config, so I passed everything through environment variables.
Use this wiki if you are lost: https://github.com/qdm12/gluetun-wiki/blob/main/setup/providers/custom.md#wireguard
It output in USA for me, it torrent with a very decent speed, 150-200/Mbps.
And yes, it's free.