Judging from the pics, your gerbil is either having some pretty bad allergies or is rubbing their nose raw against something. Unless you're seeing it bump it's nose a lot against corners or bars, I'd be betting you're dealing with severe allergies.
That 'BestPets Compressed Shavings' bedding is hard to find any detailed info on! Seems like it's a UK-brand, so I'm only finding mentions of it on UK pet sites, which all just list a price and how many kilos it is.
The only mention I found was saying it's "good quality softwood". Going by the technical definition of softwood, it's probably cedar or pine, both which are bad. Other products from the same brand (BestPets) mention a 'pleasant pine scent', so I'm pretty sure that's going to be pine bedding too.
Aspen is classified as a hardwood, so I'm pretty sure it won't be that - and considering how many problems pine and cedar cause, the bedding is probably at least contributing to the problem, even if it's not the sole reason. I'd try getting aspen bedding, or even corncob bedding if they sell it around there. It might cost you a bit more than pine or cedar, but you won't be causing respiratory problems for your pets. It's shameful that stores are even allowed to sell aromatic wood bedding when it's known to cause problems.