It depends on your definition of P2W, GI is mainly a single player game and I'm sure you can clear everything for free, eventually. They've given out Noelle, Barbara, and Xiangling and will give out Fischl next month. And you're guaranteed to get 4* and 5* characters with enough time.
Additionally, with the way damage formulas work in this game, most of your power comes from weapon/artifact stats and simply triggering elemental reactions. That means you can build one carry unit with good gear and use the other units just for their elements. And unless it's a timed challenge, you can kill anything if your execution is good, since you can perfect dodge.
That all being said, this is a gacha game. You will hit a wall where free primogems start to dry up and your progress will slow down if you aren't using resin refreshes. They frontload a lot of free stuff, but when you get deeper in, everything requires more and more resources and premium currency dwindles.
Now, after you get to that point you can just play casually and progress at your own pace, do dailes, login for events, etc. And for a lot of people that'll be fine. Casual maintenance mode until the next content drop, save gems for banners you like.
On the flip side, I'm sure there will eventually be ranking events, and probably even PVP. Nearly every gacha does them, and they're almost always P2W, with whales always getting the best rewards.