If I was going to play a gnome paladin of conquest in a serious game, I'd play him a as a bit of a sociopath. Not a full blow psychotic or casual murderer, but he'd be a zealot that doesn't let empathy or compassion get in the way of what must be done. It's not that he takes pleasure in it, but has an iron conviction that what is right must be done no matter the cost.
Now, paladins of conquest seem very Lawful to me, while there are so many gnomish interpretations around that there's no "one right way". I'd go for one of the more craftsman, everything-in-it's-place philosophy, less fey, trickster or chaotic. If you're going rock gnome, one quirk could be to make wind-up toys of people (marching soldiers, twirling ballerinas, etc.) in their spare time and lament that beings in the world can't be as orderly.
If you wanted got go more emotional, put out a dislike of the larger races that keep the gnomes in check through their size-ism. But with the medium sized races it's just a basic dislike that can be overcome, getting more intense the larger the race. That last part is to make sure the character can still effectively play with the rest of the party.
Oh, and they need a rapier. Nothing is better than a gnome paladin with a rapier, right [MENTION=6799753]lowkey13[/MENTION]?