TwoSix
Everyone's literal second-favorite poster
Depends. I'd have to evaluate a few things. Is the player interested in the gnome mechanically or thematically? I have no problem reskinning a gnome as a halfling, for example, if the player is primarily interested in the gnome traits. If I'm getting a vibe from the players that they're not particularly into the setting (maybe besides the gnome guy, everyone else is taking purely generic characters and nothing Dark Sun specific), that I'd probably just suggest moving the campaign in another direction and out of Dark Sun.I sympathise, but is that a battle worth fighting? If the player chooses to walk away from the campaign because he can't have his gnome, then that still leaves you with one less PC building up those themes and tropes.
I know out of the players I have that are also my friends, none of them would be insistent on playing a particular character type if the DM isn't on board. And if I'm playing with someone I don't know well, that sort of intransigence just suggests to me we wouldn't be a good fit. As a player in a new game, I bend over backwards to make sure the DM and I are on the same page with any character I make, and I would expect the same courtesy from a player joining my own game.