It depends on the type of model and amount of vibes firewall will see. On electric models, no hesitation on any size motor....vibration should be minimal
On jets, glow and gas, gorilla is fine up to at least a 50cc sized model or equivalent turbine. It's the execution of the installation rather than strength tho. Don't use water to kick it off and make it foam. Well foamed will not be as strong as minimally foamed or not foamed. Must make sure that the glue (true for epoxy also) is allowed to penetrate into the wood and that the gap is minimal.

For firewalls, any glue will fail in longitudinal tensile and in shear. That means that the glue joint will be both, pulled apart and sheared. Gorilla PolyU glue has good enough properties in both directions for at least a 50 cc engine. Epoxy will be strong enough for any displacement we could use in model planes. This excludes most 5 minute LHS epoxies tho....

As a rule, I don't use Gorilla on firewalls even if it's strong enough; I trust it fine.....it's just old habit. I've been modeling for nearly 45 years so that old habit is hard to break

I don't hesitate to use Gorilla everywhere else on a model, including joining wing panels. I use Gorilla's foaming to advantage when gluing in wing tube sockets into foam for example. Very sturdy, filling small gaps better than filled epoxy. Lighter too