The first two levels are near Jupiter.
It is established that Earth exists in this universe (something a game on a Nintendo console couldn't have gotten away with), and here's the evidence pointing to it. The opening Space Zone is in a ring around a gas giant, which is closer in color to Jupiter than to Saturn, and the stage heads from there to a Floating Continent a similar size to one of Jupiter's innermost moons. The second stage is a Slippy-Slidey Ice World with water only at the very end. The steep peaks fit well with Europa's chaos terrain and water only existing at the very end of the stage fits with the coexisting theory that Europa's ocean is only very thin. Even if near-lightspeed travel has arisen at this point in The Future, the ships would be so big the Asteroid Belt would look like an Asteroid Thicket and stage 3 was probably a Gravity Screw designed to bypass that.Diplomacy in this universe has reverted to pre-modern conventions.
We see the inhabitants of the space colony calling Gulfer a terrorist group, and then a member addressing their leader as "Your Majesty." Before the modern world, major empires (such as Egypt, China and Rome and its successors) thought of themselves as the only governmental authority in the world and called all other peoples "barbarians" and didn't recognize their leaders as diplomatic equals, even when the other guys were obviously more civilized.Gulfer are machines/cyborgs.
Among the humans we see, strange hair colors are common, and given the theory about Jupiter it could be a side effect of centuries of mild radiation exposure. Gulfer may have been founded while humans still lived on Earth, unaffected by radiation, and kept their "hair" color (conspicuously devoid of unnatural ones).Alternatively, Gulfer are in fact humans.
Going off the second theory, many ancient peoples didn't even recognize all other humans as human, a perception that persists to this day in real-life examples of Son of an Ape. They would still have been unaffected by Jupiter's radiation.Gulfer is short for "engulfer."
Star Wars had been out for a while, and the obvious naming for Darth Vader was well-known as "invader." (The other meaning wasn't yet It Was His Sled at this point, and Darth Sidious's similar naming wasn't known). Gulfer might not just destroy the environment by polluting it, they could be stripping it of its resources (especially if the human theory is true).