What will happen? Probably nothing, to be honest. But it's not a good idea to modify the permissions of system apps, even if you're allowed to change them.
Permission to sensors is pre-granted because (from the website):
To avoid breaking compatibility with Android apps, the added permission is enabled by default. When an app attempts to access sensors and receives zeroed data due to being denied, GrapheneOS creates a notification which can be easily disabled. The Sensors permission can be set to be disabled by default for user installed apps in Settings ➔ Privacy.