Borderline-old thread now, but since there's no other response...
Generally, you shouldn't need the Gain filter. If you've set up everything else correctly, then it should already be at the correct level by the time OBS sees it.
---
Games and other things that come from inside the computer, should themselves be set to 0dB to use a technical term, which means no-change, and is usually 100%. Because it's no-change, it won't clip unless the source already is, and if it's too loud in OBS, you can turn it down there. If it's too loud in your speakers or headphones, find a way to turn *that* down independently, which might mean buying different speakers or headphones, or something to go between them and the computer and has that control.
---
Mics should have a gain adjustment on the mic itself, or the physical interface that the mic plugs into. Use that to get a good level to OBS. For those that don't, but are built into the computer or use a 3.5mm plug directly into the computer with no controls on the mic, use the audio driver's mic controls to set its level, not OBS. The "boost" control (or whatever the coarse one is called) probably controls the physical amplifier, so use that first, then fine-tune with the other one that is probably a digital gain later.
A general rule is that you want as much of your gain as possible - preferably all of it - right up front, to give the least electronic noise. OBS is fairly late in the chain, so it's not the place to put that gain if you can avoid it.
Professional mixing consoles typically have about 60dB (1000x) of gain available as the very first active thing that they do, should the engineer need that much, and absolutely everything else after that expects the standard signal level with no easy or direct way to increase it. Some of processing just happens to include its own gain control, but that's to compensate for a side-effect of what the processing does, not to fix a bad level coming into that processing.
