It doesn't measure just defensive results in any world, unless you believe that goaltenders don't exist. It is a combination of defense and goaltending.
Is there anything simpler than this?
How do we measure goaltending?
By how many shots they stopped right? Save% is the most sited stat when discussing goalie effectiveness. Almost everyone serious gave up on GAA due to the fact it didn't take into account the D in front of the goalie. Goals Saved Above Average (GSAA) is an improving stat used to try and improve on save% but save% is usually pretty accurate IMO.
How do we measure D? By how many shots they give up is most common but scoring chances against is gaining popularity and the Expected Goals Against (xGA) stat is an effort to combine all the underlying stats into a single stat that will describe defensive performance.
Goals against is the result of team D and goaltending performance. It ain't rocket science.
Using GA to pass only judgment on team defence is just silly.
I guess the Blues had one of the worst D in the NHL the first half of the season before they magically became the best in the 2nd half the year thay won the cup? Binnington had nothing to do with it?