Never mind, one extra reboot fixed it. Leaving it here anyway as a cautionary tale.

How do I fix Gallery? It appears empty, with the Photos and Videos icons greyed out. The Screenshots icon is active and I can tap on it to enter but it appears empty. The files are there and opening them by some other means (such as from File Browser) works. I tried resetting the media database from Utilities and rebooting a number of times.