RE: Preserving Gorilla Glue?
For years and years I have preserved open cans of paint by taking my propane torch and directing a stream of propane gas into the open can. Propane is heavier than air and will settle on the top of the cans contents, shielding it from oxidation.
I do not know if this would work with PU glue or not, but that is what I would try.
Tom