I just started using Gluetun and it is working for a few dockers, Radarr, Sonarr, Nzbget,  however I can not access the Sab gui.  I set it up just like I did with the other dockers, and when I go to local:Port for Sab I get nothing,  I have tried changing the ports in both Sab and Gluetun for Sab.   I changed Sab back to a bridge connection just to test and the docker works fine there.

Look for the support thread for that container:

 

I ended up having to remove the docker and app data folder and reinstall to get it to work