GeForce NOW is a gaming streaming service which I recently signed up for. All in all, I highly recommend the service with a few caveats.
In order to play a game, you have to buy the game from a digital store like Steam or the Epic Games Store. I actually really like the approach since there are multiple stores and you ultimately own the game. There is competition between the stores and there are discounts (eventually) available for games. Most importantly, if you decide to leave the GeForce NOW service and buy a capable PC, then you can play your games. Save data is synced via Steam Cloud or other services. There is no lock-in to the service. Other services like the Xbox Gaming Passes, the PlayStation Plus tiers, or Apple Arcade are "renting" games which may disappear from the available catalog at any time.
The major caveat is that GeForce NOW supports a limited number of games. Today, there are supposedly over 2,000 supported games, but I guarantee there are more than a handful of unavailable titles which you may want to play. The reason seems to be a licensing/permission issue since the service is technically capable (spec wise) but publishers/developers do not want their games to run on the service. Some publishers/developers have their own streaming services or maybe they do not want to add a support burden for their company, but for whatever reason, there are many games (new and old) that do not support GeForce NOW.
When the service does support your game though, the service is pretty amazing. If you have a fast consistent low latency connection, games in 4K with full visual fidelity are very playable. It supports playing with dedicated apps on a Mac/PC but can also be played with just a web browser. Unless you are a professional competitive player, I doubt you can tell the difference between playing locally or via the streaming service.
The price is fair at $20 for the Ultimate Tier. With a discount at $99 for 6 months, the service cost is $200 a year. Considering the cost of a graphics card at the same level as the Ultimate Tier service (currently a 4080 equivalent), it takes years to justify owning the graphics card versus just paying for the service. Of course, owning a PC comes with more benefits than just games. However, I would strongly consider buying a cheap PC even like a Chromebook and paying for the service versus buying a good but more expensive gaming PC.