To be honest, if you don’t know how to install or use that command, then it’s probably the best just to upload the logs via the app. That is the easiest access to help, and, the dev can see exactly what is happening.
Gathering logs could not be any easier, so I’m not sure why you are not doing it 
If you cant upload via the app for some strange reason, then upload somewhere else and paste the link here.
To recap, so this thread doesn’t get too out of context:
- Nobody can help you further without at least logs replicating the issue.
- Symfonioum will apply replaygain if tags are present
- Your tagging software might not apply replaygain tags if tags with 0 value are already present, this also results in 0 replaygain applied on app side.
- Check out the search function in this forum, because there are many handled issues regarding replaygain not being applied.