Source: Thanks to whyitskai for information and images.
No Man’s Land
The official name of the planet in Trigun is No Man’s Land. There are variant spellings such as Noman’s Land or Nomansland (or even the odd typo Normansland on the magazines and money in Stampede). However there is a popular alternate name that is even used in some official sources: Gunsmoke.
But the name Gunsmoke never actually appears in any Trigun canon, not even Trigun (1998). Not in the original Japanese or the English translations. So where does it come from?
The Planet Gunsmoke Toyline
The first use of Planet Gunsmoke is in March 21, 2000, with the release of the Vash figure for The Planet Gunsmoke toyline.
Many people think the video game Trigun: The Planet Gunsmoke was the source, and that the toy line was made from the game. In March 2002, a 20-second teaser trailer for the game, in development for the PlayStation 2 by Red Entertainment, was shown at Sega’s GameJam event in March 2002. The game never left development. But most importantly, the toy line came out in 2000, two years before the teaser trailer. YouTube channel TheMonit0rLizard produced an episode of Lost Media Monday on the game and its history here:
The last episode of Trigun (1998) aired September 30, 1998, two years before the toy line. So why do people think that Trigun (1998) is the source?
On February 23, 2005, Dark Horse released their English translation of Trigun Maximum Volume 4: Bottom of the Dark (originally released on September 1, 2000 in Japan). Each collected volume of Trigun contains a bonus comic by creator Yasuhiro Nightow where he discusses behind-the-scenes aspects of Trigun. In this particular comic, he discusses the creation of the Planet Gunsmoke toy line.
“First off, the difference from the Trigun manga world and the Trigun anime world, I believe it says “The Planet Gunsmoke.”
And here is the problem. The translation is phrased confusingly. He seems to be saying “The anime uses the name The Planet Gunsmoke and the manga does not.” In fact, he is saying: “UNLIKE the anime and manga, the toy line uses the name The Planet Gunsmoke.”
Dark Horse compounded the error on the back of their release of Trigun Maximum Volume 8: Silent Ruin on February 1, 2006. There the description of the volume says Knives will not “have his way with Planet Gunsmoke!”
Since then, American distributors and merchandise creators picked up the name Gunsmoke and use it with some frequency, including for Trigun (1998). Clearly it’s a good name for marketing and has more thematic associations in American culture than No Man’s Land.
So Is It Canon?
Planet Gunsmoke straddles the line of canonicity. It has never actually appeared within Trigun itself, but Nightow himself created the name for the toy line, and Dark Horse appears to have popularized it in their marketing material. Many years later the name is used widely in fandom and marketing. Whether you use Gunsmoke or No Man’s Land is up to you. But we know the source of the confusion.