I do not know the composition of gerbil urine

[bunsen_h]

But on drying, gerbil pee leaves a tough white residue, poorly soluble in water and difficult to remove from a plastic surface without damage to the plastic.  However, it is readily soluble in concentrated hydrochloric acid, with evolution of gas.  I suspect it's giving off CO₂, but I don't have the resources to try to confirm.  It would probably also dissolve in something like vinegar, but much more slowly.

I miss working in a chemistry lab for many reasons, and one of them is ready access to small amounts of harmless chemicals for personal use.  But some reagents are available from craft and hardware stores, if you know what to look for.  Hydrochloric acid is also known as muriatic acid, and the version sold in hardware stores is pretty close to the concentrated HCl we used in labs.