You'll need to log into Google Fi to activate your service. However, you shouldn't need to remain logged in after the eSIM is provisioned. So, you could theoretically set up Google Fi and activate service, then perform a factory reset (I don't think GrapheneOS clears eSIMs during factory reset).
I used Google Fi with the stock OS. After flashing GrapheneOS, I was able to use the previously provisioned eSIM without installing Google services.
I needed to switch plans and accounts recently so I had to install Google Fi (and it's dependencies). This also required deleting the former eSIM (iirc, Settings > Network & internet > SIMs).
I cannot speak to how it handles international travel when not logged in. The occasions that I've traveled internationally, I had Google Fi installed and authenticated.