To allow GrandPerspective to scan as much of your disk as possible, you should grant it Full Disk Access. This may also resolve hangs while scanning.

On start-up, GrandPerspective performs a check to try to establish if it has Full Disk Access. When that does not seem to be the case, it shows a dialog to notify you. From there, you can go to the System Preferences where you can grant the required permissions.

Granting Full Disk Access can be done via "System Preferences" > "Security & Privacy" > "Privacy". In the permission pane at the left, find "Full Disk Access" and select it. Click the lock to make changes. If needed add GrandPerspective to the application pane at the right; you can do so via drag and drop. Tick the checkbox to grant permissions.

Even with Full Disk Access enabled, GrandPerspective cannot read all files. In particular, it is not able to scan the following:

• Files owned by other users. These can be scanned by letting their owners scan their home directory themselves
• Certain system files. Data Vaults, for example, are directories that are only readable by a process that has a specific entitlement. Being root will not give you access, nor will enabling FDA.