I think a lot of folks are associating GaaS with multiplayer centric emphasis. Isn't a GaaS game simply a game that is continually offering new content? Not like a game simply with DLC, but a game that offers more and more content over a greater period of time. No Man's Sky comes to mind. Sea of Thieves. A lot of MMOs, I would think. Halo Infinite. Marvel's Avengers. As you stated, Outriders doesn't have any new content at all. So assuming my understanding of GaaS is correct, I agree Outriders doesn't fit in that mold at all.
It's a somewhat fuzzy term, but if you aren't continuously adding paid content, you aren't GAAS. Outriders can't be... really No Mans Sky isn't either.
The key is a game built for a continuing revenue model. Subscription based games are the most obvious, and the original GAAS games. World of Warcraft being the most famous.
Then the industry realized that model wasn't going to really work for most games; for a number of reasons, but mainly because.. most gamers dont want to subscribe to a service for a single game.. not directly anyways, the psychology of it doesn't work.
But the industry still wanted a lot of "post purchase revenue".. so we got:
1) MTX, horse armor! doesn't make you GAAS either, but these were introduced more to pad the price of a game
2) Pre-defined DLC, nothing really new.. we've had "DLC" since the 90s.. but what they did was started promising a specific set of DLC before the game even launches and then letting you buy the "Season Pass" or "Ultimate Edition" or whatever.. again not REALLY GAAS.. but we were getting there.. these again were more methods to pad the price of a game..
3) The industry then went "aha!! what if we just endlessly release DLC and MTX..." and modern GAAS was born... early innovators realized there was so much money in this, you could even release... your initial game.. FOR FREE
4) This has been refined over the years, now we have "Seasons" and "Battle Passes".. these are KINDA like the "Season Passes" of "get all the DLC at once!" approach, but they are designed to happen constantly and consistently, usually roughly quarterly..
5) Games in this genre rarely if ever get sequels.. because why would you need a sequel? You are getting sequel money every year from some of your players.. some? Every quarter.. or month.. or day... again, most famous of these "games that never get sequels" being.. World of Warcraft... approaching it's 17th birthday.
What you are left with is games that basically act like subscription service games.. but hide it behind a bunch of other carrots on a stick.. and also hide it by making it POSSIBLE to just buy the base game and play. Of course w/ a year or 2 half of the content won't be available to you lol