There are two realities: Gundam Wing and Steampunk Era (for lack of a better name). In Gundam Wing, Professor G survived the explosion (don't know about the others) and is working on something new: an acrotoggle. If perfected, it will cure cancer, starvation, and red eye. Okay, it will get G a lot of money, and he's hoping it will be a transporter—but J already called dibs on teleportation, so G had to name his device something else. For this reason, he gathers the characters in small groups in various places. They most likely don't know that G gathers them in these places; they're gathered to whatever place on some ruse.
Meanwhile, in the Steampunk Era, Isis Galen, world-renowned alchemist, has also been working on something new: a potion that will grow money, turn water into vinegar, and bleach teeth. More importantly, it will allow Isis to escape Lady Dorothy, daughter of his current patron. To test it, Isis secretly uses people who annoy him the most, i.e. the AU-counterparts of our characters in Gundam Wing. The potion won't work until Isis says the correct incantation (Isis has dabbled a little in magic, a "lesser science" in his mind). He manages to get the potion on all of the characters needed.
If there were pictures, we would have a split screen now with Professor G activating his secret invention and Isis Galen reciting the needed incantation at the exact same time. The acrotoggles explode something horrible, and if you know Isis, it's no surprise that his potions have the horrendous effect of also exploding somehow. What is surprising, though, is what effect these simultaneous explosions have. Somehow (through fake science that we won't bother to explain), the explosions rip the fabric of space or something. The characters in Gundam Wing are flung through the holes and into the Steampunk Era. We can assume that the AU-counterparts are flung through the holes into Gundam Wing as well, but we don't care about them.
In this new place, the characters now have the option of taking on their counterparts' lives. If they don't, they will probably still have to deal with things. This new place does not have electronics or our modern technology in the same sense. They have computing devices which use cogs and gears and perhaps even steams. Steam engine trains are the main use of transportation across long distances. Many people still use horses and carriages frequently (assuming they can afford them), even using these as an alternative to trains at times. Some crazy people are trying to invent a horseless carriage, but so far, their attempts have proved fatal.
There are no space colonies here, for people haven't left the Earth's atmosphere. Instead, there are a five large colonies floating in the sky, accessible only by airships. Most people on the ground only see airships; only the lucky few get to ride on one. Most airships are under control of a military, pirate gang, or wealthier merchant company. There are very few privately owned ones, and there are no real "luxury cruises" in the sky just yet. This is partly due to the fairly large number of pirates. Arrgh!
Life in this place looks a lot like the Victorian Era (as you might have guessed). Fashions are based on the time period (with alterations), many social norms (though not all) are in place, and speech is slightly more formal (though as Trowa will surely demonstrate, this may be the first to go *huggles E*). Also, as you see by the map, the countries (aside from Cinq) are not the same.
In my fic, I have it that Dorothy (a year older than the pilots, Relena, etc) is about to turn 20, the age at which ladies traditionally marry. I don't know if you want to pursue that in this RP or not. It doesn't much matter to me because I'm writing the fic, so it's up to the rest of you if the girls are having societal pressures to marry or not (could be fun and wacky in the right hands, but could also be easily ignored ^_^).
Um, that's about it. If anything needs clarification, let me know.
Edit: I can also generate a town map if needed for any town; just give me specifics (will let you know what those are if anyone wants to know). I have a nifty neat generator thingie.