Originally posted by dirktoot:

Gjalls don't destroy items. Nothing does. What does happen is that items despawn if not in range of a workbench. If the Gjall destroyed your workbench too, and it took you more than 2 or 3(?) ingame days to come back to that area in order to pick up the mats then yes they are gone.

If it has been less that it's probably a bug though.