Gralats will disappear if they haven't been updated in a few days, probably to reduce the number of entities Graal has to keep track of. One Xmas I farmed on another account for a while and then when I logged to unlock them for my main, they had gone. This ties in with the fact you haven't been into your guild house for a few days, and the period is maybe decreased as its very laggy at the moment.
This has been a very rare occurrence and An unfortunate part of the game - to my knowledge - for some years now.