For Beginners - How-to SSH to RPi - Setup Putty in Windows
For Beginners - How to Set / Change Gain
But changing gain depends on what image / decoder you use.
Did you run one of those two, then there are shortcuts to change gain:
Automatic installation for readsb 路 wiedehopf/adsb-scripts Wiki 路 GitHub
Automatic installation for dump1090 fa 路 wiedehopf/adsb-scripts Wiki 路 GitHub
You really need to provide some info on your setup when asking these questions 