When wet, the hydrogen bonds between the cellulose chains in paper are disrupted due to the formation of hydrogen bonds with water. This leads to the paper becoming extremely weak. This is the case with most paper, however, additives such as gelatin and aluminium sulfate are added to the paper to strengthen it.
I can't see how the addition of either of these strengthens the paper. I would guess that it aids the bonding between cellulose chains, perhaps by creating a 'bridge' between them, but I'm not sure if this is correct or how it would happen. Another idea is that the aluminium could form a complex ion, and then perhaps form a chelate to strengthen the paper, though I have little confidence in this.
So, how do the gelatin and aluminium strengthen the paper - particularly filter paper, which is almost pure cellulose.