Hi!
I have used Polyurethan glue (Think it's the same as what you call Gorrila glue) for years om my pylonracer wings.
It's much better than any spray contact glue you can find.
Just spread out a thin layer of glue on the balsa sheet (you have to tape together the balsa sheets first) and then spray some water mist on the sheet and finnaly put the sheet in the foam core and lay it between two thick wooden peices and use some sturdy clamps to press it all togheter.