The same problem is noted here: https://forum.affinity.serif.com/index.php?/topic/43084-affinity-photo-unable-to-import-psds-from-google-drive-on-ipad-pro/?hl=%2Bipad+%2Bgoogle+%2Bdrive
It doesn't seem right that Affinitiy Photo has zero permissions. When checking iPad -- Settings -- apps - Photo, there is just nothing there. It's the only app on my iPad that looks this way.
FWIW, I remember a month or two ago trying to open RAW files from Google Drive and then too getting only grayed-out options.
PS All these are duplicates:
https://forum.affinity.serif.com/index.php?/topic/41573-raw-import-from-google-drive-not-working/?hl=%2Bipad+%2Bgoogle+%2Bdrive
https://forum.affinity.serif.com/index.php?/topic/41702-opening-raw-files-from-icloud-or-google-drive/?hl=%2Bipad+%2Bgoogle+%2Bdrive
https://forum.affinity.serif.com/index.php?/topic/43081-unable-to-import-raw-files-from-the-cloud/
https://forum.affinity.serif.com/index.php?/topic/43084-affinity-photo-unable-to-import-psds-from-google-drive-on-ipad-pro/