PSxG is a metric similar to xG. But while xG takes into consideration only the position when the shot happened, PSxG takes into consideration also where a shot ended. For example, a penalty taken by player A and B have the same xG. However, if player A shoots it in the corner with a lot of power, while player B shoots it into row Z, the PSxG is very high for the first one and very low for the second one. Same for shots everywhere else in the pitch. So, in many ways it is better than xG.
PSxG-GA essentially is the sum of PSxG minus the goals a keeper conceded (GA = goals against). For example, we remember De Gea in God mode in 2017-2018. He faced 39.7 goals worth of PSxG. However, he conceded only 28 goals, almost 12 less than expected, so essentially, we conceded 12 goals less than if we had an average goalkeeper.
It is probably the best metric to judge goalkeeper's shot stopping ability. Of course, if you want to have a statistically-based opinion on who are the best keepers, it needs to be complemented by statistics that account for passing and dealing with crosses.