I am not sure there is a flaw, looking at raw GSAA from different season is probably just exactly like looking at raw points between different season.
The value of a goal scored change a lot, same for a goal saved. No one would ever even just think to look at raw point total between a 70 games seasons in the 50s and one in the 70s, same goes for GSAA
I should have clarified. It's flawed in the sense that we need to consider the context before accepting the numbers at face value. (This conclusion will be obvious to most of us, but it's worth emphasizing).
Some of the issues that need to be considered:
1) the quality of the league average goalie - this was obviously significantly higher in 1967 compared to 1977 (for example), which makes it easier for a top goalie to beat the league average by a wider margin in the 1970's.
2) how goalies are deployed - GSAA is essentially combining save percentage with workload, but a goalie's workload is partly dictated by leaguewide coaching preferences. Goalies who played in an era where coaches used a "platoon" approach will have their GSAA deflated, even if they played a ton of games relative to their competition at the time (ie Patrick Roy)
3) length of the schedule - all else being equal, it's easier to record more GSAA when there's an 80+ game schedule compared to the shorter seasons in the Original Six era, right after expansion, and in the lockout/COVID years
4) flaws inherent in save percentage - I won't both repeating them here (I've covered this is numerous other posts over the years), but any flaws in save percentage are baked into GSAA
Despite these issues, I think GSAA has some informational value. But simply comparing the numbers across different generations can lead to flawed conclusions (is anyone going to argue that Vachon, Joseph and Vanbiesbrouck peaked higher than Hasek, Roy and Plante)? I realize I'm probably stating the obvious here, but skimming through this thread, I already see GSAA being quoted without any context, so maybe there's some value in talking about this upfront.