You shouldn't have to screw with OBS' audio settings.
Baseline your audio playback and don't touch it.
- Go to Windows sound control panel and for each audio device enabled, set the audio level and vow to not touch them. Everything from there is referenced off the level you set. Set it to 90 as the highest limit.
- For each application, control gain inside the application's audio settings if it has options to do so
Use Windows Volume Mixer. You will be able to control gain at the device and application level.
None of that needs a OBS gain filter.