FWIW, I tried a foaming/expanding polyurethane glue on a balsa/depron fus... unfortunately I didn't spread it out well enough and where it was thickest in the middle of the fus it expanded BIG time! I had sheets of 3/8" glass on top and then 1/2 doz house bricks for weight. When I made the cut-out for the wing I found gaps of ~1/16" where the glue had 'bubbled' [
] Strike One! On my second attempt I used a very thin skim of 30min epoxy
I'm not saying don't use PU glue, just make sure it's a real thin, even layer you're putting on.
(BTW the poly glue I got here in the U.K. is like the thickness & consistancy of silicon sealer and it comes in the same type of industrial tube. If Gorilla is thinner then you'll have an easier time).
HTH,
Steve