Cool stuff, TIL gluetun. thanks for sharing.

To my understanding according to your docker-compose, since your nginx and gluetun containers are in the same default compose network, I believe you can drop the ports part from your gluetun container;

  gluetun:
    image: qmcgaw/gluetun:v3.41.3
....
....
    ports:
      - 8080:8080

Nginx container should have no problem reaching to gluetun:8080 using the container name as DNS record.