GrapheneOS puts Google apps in a sandbox so they don't have access to most the information they do on stock Android. Combined with putting it on a seperate user, it severely cuts back on the amount of information it can access.
Privacy and security exist on a spectrum, nothing is absolute. GrapheneOS gives me 99% of the functions of my phone, while greatly enhancing privacy and security. If I were more paranoid I wouldn't enable the Google tools, and have less function, but maybe more privacy. Everything in life is figuring out what trade-offs you want to make to get what you want.