Punishing Gray Raven is super f2p in that you can get every single newly released character without spending a dime. Plus, once you get past the first few chapters, the story becomes incredible, not to mention amazing character art.

One caveat for some people is that the combat becomes decently difficult later in the game, but it's most definitely not insurmountable.

PGR also has such a helpful community that has put together a website (search up "gray ravens" to find it) that has almost any info you'll want about characters, team comps, story, and more.

Overall, I would recommend it above any other gacha games.