I'd bring the gnome in from another plane (or crash on a Spelljamming ship). The reason I can do that is because all of my D&D campaigns are theoretically set in the multiverse that includes all of the other official settings that were around at my point of timeline alignment.
Personally, I would find a planar glitch mode of incorporating a PC, and even moreso a spelljammer mode, as extremely destabilising of any campaign world (including Athas/DS) that I could imagine GMing. It suggests that the world where the events of the campaign will actually be taking place (that little slice of geography and history) is not a very big thing at all, and that the real stakes are elsewhere.I agree with the spelljammer/planar travel aspect being the simplest and most seamless way to incorporate the character. It would have the least implications for the established history of Athas.
Generally I don't bother with gnomes or halflings. To me, they don't carry any of the weight that elves, dwarves and orcs do. In the last long-running GH game I GMed I treated all references to gnomes and halflings as indifferent references to a particular, slightly smaller, variety of dwarf. In my 4e game I think one halfling NPC turned up early in the game (I had prepared an encounter for the first session when I expected one of the players to be playing a halfling, but at the last minute he changed to a half-elf), and I used a couple of gnome antagonists at one point also fairly early in the game.
If, for some reason, I had to incorporate a gnome into my game, I would generally take the player's lead on lore, given that they're the one who cares enough about gnomes to want to play one in the first place. (Eg in an earlier GH game a player of a gnome modelled them on the Petty-Dwarves that occur in the Silmarillion.)