Bittersweet Ending — The war is lost, at least in the sense of the Parts Kingdom no longer existing, even though the Motorouin leaders have been quashed. But the characters survive and move on to other things, or get married and prepare for the next war.
Child Soldiers — Rivet is the oldest player character by a whole three years, and she is only 18 years old. Then again, the very theme of the game is the conflict between the vital, change-capable youths on the one hand, and the immortal, stasis-craving elders of the Motorouin on the other.
Demonic Dummy — The boss of stage 3. "Hey! Stop trying to hide behind me!"
Face Death with Dignity — The final boss shouts, "I do not fear death!" In the first loop, anyway, and that's probably a reference to his wired-in immortality. When felled in the second loop, he just screams in horror and pain as Volvox blows up around him.
Our Presidents Are Different — The president is a lion. Actually, he has a lion's mask, to conceal his actual wired-up face, which you'll see on the second loop. Drill must have had problems getting the immortality tech to properly take.
Screwed by the Network — The rights to this game and all related assets are owned by Capcom. This means that Cave cannot make any updates, re-releases, ports, or otherwise.
Theme Naming — All of the player characters are named after components that are used to hold things together: Ring, Bolt, Chain, Nail, and Rivet. The bosses are named after tools. This also extends to the organization the bosses comprise, Motorouin. "Motor", but it's also Japanese for "senate". And then there's the name of the game—"gear", but with the "pro" at the beginning it sounds a lot like "progeria", a medical condition in which the victim ages particularly quickly. This is probably meant to evoke the game's theme of youths being forced to adopt mental maturity much earlier than usual.