GrapheneOS currently doesn't ship Play Store or Services OOTB. They install as normal apps (albeit with GrapheneOS providing support code to make the fact that these things use/expose custom privileged APIs work correctly). I don't know if the Google TOS would prohibit that, at least I am not aware of any enforcement action against GrapheneOS in this regard. GrapheneOS also doesn't have Google's blessing, meaning some apps like Google Pay won't work on it. To get this, you need to apply to be an OEM.

Really Motorola doesn't need to sell a GOS phone. Motorola just need to sell a phone with the right hardware security features, open source/upstream their Android/Linux patches, and give users the ability to run GOS.

Hopefully they can then give you the option to buy one with GOS preinstalled, but even if they don't. It will be sufficient that it can run GOS.

Unlike Windows, nobody feels they're paying an inherent tax when buying a stock Android phone. I'm sure nobody will mind.

The hard part will be actually supporting the phone for long enough.

GOS is reliant on Google's open sourced Pixel android releases up to and including the 9 series. This is because GOS doesn't have the resources to handle that entire side of things. But I guess part of that is also that GOS doesn't have access to the necessary information to do that stuff properly either.

That's fair, this would still be a valuable development even if Motorola doesn't end up shipping devices with GrapheneOS preinstalled, but if they did I think there's a lot of potential for them to enter the mainstream. A device with GrapheneOS without any [major] caveats (like Play Integrity API, Google Pay not working) would be a game changer.