My understanding is that gain of -10 is a very, very high setting that you can’t set manually otherwise (something close to 60dB, it appears your test shows it at 58.6dB). You can’t set your gain above 49.6(?) unless you have it set to -10. It’s possible you’re blasting your decoder with so much noise and such high-power ADS-B signals that it can’t read the signals from many other aircraft.
So you should set gain at something in the middle (around 25 or 30) and then test increasing and decreasing the gain once per day to see if your performance improves.
This post has a list of the allowable gain settings.