I am lucky in being able to tame the nasty side of gloppies for my cyanotype sizing works. I inherritted a whole ton of organic and inorganic chems from a biotech r&d lab after a bankrupcy interruption in operations.
I have a jar of very carefully stored, pretty potent, biocide that I mix (under very carefully controlled conditions) as a 1% solution, and then mix that as a 10% solution so I am down to a 0.1% solution. Add 20mL of that stuff to the unhardended gelatin, stir it in, and put the gelatin in the fridge.
14 months later, and there is still no sign of ucky goo ilike usually loves to grow in the gelatin.