Piggies sometimes sneeze if they get in their nostrils.Check your hay and make sure it is not dusty.Of course they can sneeze if they have an urti but there would be other symptoms like runny nose/eyes and the piggy would appear unwell.I do not think you need be concerned about Grayci,just make sure there is no dust and nothing that could cause an allergic reaction(household sprays,in fact,sprays of any kind can cause sneezing).
I will have a look at Peters website to see if there is any problem.You could get in,I think,by just typing Peter Gurney Guinea Pig health into your search.
Peter wrote 7 books about guinea pigs,all of which are worth having,although the earlier ones like The Proper Care of Guinea Pigs has a lot of now out of date info. The updated version of Piggy Potions is worth having and it has some lovely,and very funny.illustrations.None of his books are expensive,round about the £6 to £8 mark.
Mary