BluishHumility This is somewhat alarming. What causes GPS to continue consuming resources if it has no permission to do anything? What on earth is it doing?
It is not alarming. Those apps on GrapheneOS have the exact same access and live in the exact same sandbox as all other apps. There's nothing special about them. That said, if you're using an app which expects to connect to the network or have unrestricted battery usage, and you've restricted that, it'll keep trying to do the things it needs to do to perform, which is likely what was causing the battery drain in this user's case. An app silently crashing repeatedly is bound to consume more battery that you'd expect.
As far as I understand, the baseline "correct" and supported setup for Sandboxed Google Play is all of those 3 apps installed, with the network permission and unrestricted battery usage granted to Play Services. According to the lead developer, this ironically makes Play Services consume less battery, not more.
BluishHumility Is there any way to further disable GPS, beyond just turning off all permissions?
Yes. On Stock OS you can only disable system apps, whereas on GrapheneOS you can disable user-installed apps as well.
Simply long press the app, go to "app info" and you'll see the disable option there.