2974 - Gim Allon is born on Mars.
2988 - 14-year-old Gim discovers the ancient Manitou Stone on Mars and gains his growth powers.
2990 - 16-year-old Gim becomes a cadet in the Science Police Academy.
2991- 17-year-old Gim joins the Legion of Super-Heroes in their first recruitment drive. He divides his time between the Legion & his studies.
2996 - 22-year-old Gim becomes a science police officer. He takes the new identity Leviathan.
2997 - 23-year-old Gim's relationship with Salu Digby ends when Ayla Ranzz joins the Legion of Super-Heroes. He steps down when Lyle Norg & Querl Dox hack the Science Police.
3002 - 28-year-old Gim becomes the delagate officer for Mars when the Manitou Stone flares, creating an entire community of enlarged humans.
3006 - 32-year-old Gim marries Yera, a fellow Manitou Martian.
You'd think there'd be more characters with the ability to grow to giant size. It's a cool power. so of course, the Legion has their own character with this power. Colossal Boy has one of my favorite original costumes (love those gloves), and I also really enjoy the idea of him being a cop first and a superhero second. That's a cool premise that just doesn't seem to get used often enough.
In our ongoing effort to give everyone more singular backstories that make them unique in the world, we actually decided to give Gim his powers from an already-established power source; The Manitou Stone. This means he has the same power source as Super Chief & Longshadow. They're magical. Given that fact, I'd like to imagine that Gim is actually of Native American descent. This means the Manitou Stone would have had to made it's way to Mars at some point, but even that doesn't seem like a huge stretch.
We didn't change much about Gim's story, just used his status as an actual Science Police Officer to better define his story. We included his early relationship with Salu Digby, but of course she winds up with Ayla Ranzz (in one of the very first gay relationships in all of comics), so we used the Legion's adversarial relationship with the Science Police to explain his leaving the team. Also, Mark Waid's run on the Legion suggested that he came from a planet of giant people, and that's kind of cool. We introduced a community of other people empowered by the Manitou Stone on Mars that he can police. It's a cool bit of character extrapolation, but also neatly expands the world of DC in the future.