The Gremlins
The Gremlins are clearly symbols, but for what? Perhaps one can see them as natural forces, like the furies from Greek folklore whose destructive force is like the decomposing aspect of nature. They are monstrous and disruptive, like chaos, so they can be seen as elements of chaos. The symbolic rift between humans and Gremlins can be seen as a man versus nature conflict where people must realign their interests to the greater good of their natural habitat.
Flight and aviation
The flight of the Royal Air Force is shown as a symbol that stands in opposition of the Gremlins natural habitat. This is a way of suggesting that human flight is something supernatural. In order to correctly harness the new power that has come through skillful human flight, the Air Force must realign themselves with the forces of nature that they transcended. They do literally fly above the natural world of the Gremlins, so that is another reason to consider this symbolic interpretation.
Hitler and the Nazis
Unsurprisingly, Hitler and his Nazi regime can be seen as the archetype of human evil. This is because the Nazi movement was one of the most horrific moments in all of Western history. In this book, they represent the forces of evil. There is a binary of good and evil, and the RAF are the good guys, and the bad guys are the mechanistic Nazis who use their power to disrupt social order, impose their rule, and kill millions of innocent people in the process.
The conversion of monsters
The monsters undergo a conversion that symbolizes something important in the story. It is the turning point in the plot. It can be seen as the climax of the whole story. The Nazis are the antagonists, of course, but in a more important way, the true story isn't a battle against Nazis; it is a battle to align the Air Force with the natural powers of destruction, which is what the Gremlins represent. They align themselves to the archetypal forces and find victory.
The allegory of false enemies
The false enemies of the story are the RAF and the Gremlins. Of course the Gremlins are antagonistic to the Air Force, but the time they spend fighting the Gremlins is poorly spent. The allegory teaches to understand priorities. Yes, the Gremlins are destructive, but if they are educated and trained, they aren't evil like the Nazis. They are just naturally troublesome. Actually, the RAF decides that their destructive tendencies might be helpful on the battlefield. They were never enemies at all, not really.
