It comes from the publishers in the 90s. They needed an easy way to tell stores/distributors how popular they thought each of their games would be, to help them decide how many of a certain title the distributor should order. The games expected to be GotY contenders would be marked AAA, AA for otherwise decent games, A for more niche games and B for “this is a starshot, we’re hoping it will sell enough to justify production costs”. That then lead to more and more games being marked as AAA due to budgets getting increased, and the whole system became a bit redundant.
As someone who’s played a lot of GW2 over the past couple of years, I can confirm that it’s still fantastic. It doesn’t get anywhere near the amount of content that WoW gets, but it’s on a good cadance these days and outside of buying expansions, is absolutely playable without spending a penny.
Helium Rain launched a few years ago as a commercial game with an open source launcher (BSD-3), and as of a few weeks ago the game became free on Steam. The developer is no longer maintaining it, but there’s still a small community that are interested in it.