A while back I posed that same questions to the Gorilla Glue mfg. Here is their reply:
"Gorilla Glue is a polyurethane adhesive, and like all polyurethane glues, reacts to moisture to cure; that is, moisture is what causes the chemical reaction in polyurethane adhesives to harden. In reply to your concerns, it sounds like the glue bottle may have been closed with the air trapped inside. When the bottle is opened and the glue is exposed to the moisture in the air, it starts to cure (harden). For this reason, properly storing the glue is critical. We advise customers to squeeze out any excess air before recapping the bottle tightly. You will need to make sure the glue is at the tip of the bottle before capping – otherwise air can be sucked back in. The bottle should be a bit misshapen if all the air has been removed. This must be done after each application of the glue. Once the glue has cured, there is no way to return it to liquid form, so please do not attempt to heat the glue in any way. Shelf life of completely unopened bottles would be about 3 years; however, once opened and not closed properly (that is, all air squeezed out), the shelf life would be very short.
Any glue that is still in liquid form is reliable and usable. Take the bottle apart (be careful not to spill any glue) and clean the nozzle using a dry cloth or a toothpick to remove hardened glue. If you have a crust on top of the liquid, we recommend poking down through the crust (with a stick, screw driver, etc.) to remove the top dry layer. If your glue is thick, you might try placing it in a bowl of warm water to thin it. However, never microwave <personname>Gorilla Glue</personname> as it can explode and cause damage. We recommend wearing protective gloves and old clothing when working with our glue.
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