My Gnomes are Different

The problem with gnomes in fantasy games is that no one can really agree on what they are. Elves, dwarves, etc. all come from a distinguished line of Tolkien-inspired literature, and so it is easy to make Pellatarran versions of them different by simply stating how they differ from the standard. Gnomes, in my experience, do not have that luxury, and their entire basis for being in D&D is that they were brought in to substitute for halflings back when TSR was being sued by the Tolkien estate.

Pellatarran gnomes are not:

• Knockoffs of halflings with some characteristics changes.
• Ridiculous tinkers, a la Dragonlance.
• Tiny folk in red hats and blue coats. 

They are instead:

• Magicians, thinkers, and philosophers who have a symbiotic relationship with humans.
• Prone to humor that ranges from wry and dry to dark and morbid to obscure and occult. 
• In short (pun intended), they are the I.T. nerds of society. 

History
An ambassador race, the gnomes were created by elves specifically to parlay with dwarves. Short in stature and comfortable underground, they proved ideal for this task, and soon became their default diplomats to all races. (Kindly note that the Bard class is ideally suited for diplomacy, especially if they choose "oratory" as their type of bardic performance.)

Considering that humans were originally created by the dwarves to be ambassadors to the elves, eventually two races began interacting with each other more often than with the races they were intended to liaise. It's not surprising that the two peoples quickly adapted to each other and formed not only an easy, effective working relationship, but also a kinship that lasted well after the two races were emancipated by their creators. 

Society

As such, humans and gnomes get along like siblings, or at least cousins: occasionally there is a squabble that results in mild violence but usually both races can live in harmony with each other. In some larger human cities, gnomes comprise half of the population, creating a race-based division of labor with the taller, stronger humans performing tasks which require height or muscle mass and the smaller, magically-inclined gnomes taking on duties which require scholarship or greater dexterity.

This is not to say that gnomes seek positions of power within society while leaving the humans to be the working class; far from it. If nothing else, humanity's dwarven origins would prevent such an unequal situation. It is more that the Two Peoples gravitate to their respective strengths, bolstering the weakness of the other. 

On the whole, humans prefer positions of physical or martial strength, or good honest labor, whereas gnomes prefer to be left alone at their desks to read, study, or administer. There are of course exceptions to these rules -- there are plenty of scholarly humans and martial gnomes -- it's just that these run counter to the stereotype. 

Gnomes and humans are known to intermarry, but are infertile.

Psychology

As race created by the elves -- who are chaotic, terrifying, as prone to unpredictable violence as the sea, and possessed by crippling phobias -- gnomes are surprisingly stable. Their magical nature and chaotic origins were tempered by the inherent purpose of their design: to interact with more boring (read: less chaotic) races. As such, they manifest their chaos through socially acceptable means: odd obsessions, strange senses of humor, whimsical decisions which make little sense at the time. Despite all this, most humans find their behavior oddly endearing, or "eccentric but harmless".*

It is a point of mild interest that while gnomes make fine healers, they are generally not drawn to the Church (which is largely a human endeavor). They are more likely to be drawn to the paths of Druidism and the Gray Cabal.

In general, while sociable, gnomes do not do well under stricture and hierarchy. They much prefer to be left alone to pursue their studies and hobbies, reporting in only when they require assistance, have something to report, or need more work to do.  (See reference to I.T. nerds, above.)

Physiology

Gnomes of Pellatarrum are physiologically indistinguishable from other Pathfinder RPG gnomes. Quoting from the Pathfinder System Resource Document:

Gnomes are one of the smallest of the common races, generally standing just over 3 feet in height. Despite their small frames, however, gnomes are extremely resilient, and not as weak as many of their foes assume. Though their diminutive stature reduces their ability to move quickly, gnomes often train to take advantage of their size, especially when fighting foes much larger than themselves.

The coloration of gnomes varies so wildly that many outsiders assume gnomes commonly use dyes and illusions to change their skin and hair tones. While gnomes are certainly not above cosmetic enhancement (and may wish to change their appearance just to see how outlandish they can look), their natural hues truly range over a rainbow of coloration. Their hair tends toward vibrant colors such as the fiery orange of autumn leaves, the verdant green of forests at springtime, or the deep reds and purples of wildflowers in bloom. Similarly, their flesh tones range from earthy browns to floral pinks, and gnomes with black, pastel blue, or even green skin are not unknown. Gnomes' coloration has little regard for heredity, with the color of a gnome's parents and other kin having no apparent bearing on the gnome's appearance. Gnomes possess highly mutable facial characteristics, and their proportions often don't match the norm of other humanoid races. Many have overly large mouths and eyes, an effect which can be both disturbing and stunning, depending on the individual. Others may have extremely small features spread over an otherwise blank expanse of face, or may mix shockingly large eyes with a tiny, pursed mouth and a pert button of a nose. Gnomes rarely take pride in or show embarrassment about their features, but members of other races often fixate on a gnome's most prominent feature and attempt to use it as the focus of insults or endearments.

Crunchy Game Mechanics

• Replace Keen Senses with Blended View.
• Replace Defensive Training and Hatred with whatever you like.

Gnomes were created by elves to be an ambassador race to the dwarves, so having both low-light and darkvision is reasonable. They also don't hate kobolds like traditional gnomes or go to war with giants; most Pellatarran gnomes live with or near human settlements, so pick something that replaces defensive training and hatred to make your gnome more interesting. 

Gnomes are generally not Lawful in alignment, although there is no in-game mechanic for this. A good reason for why this character breaks the mold would be good to have, though. 

Relations with Halflings
Gnomes are also known to cohabitate with halflings, although their similarity in size usually results in a more equal division of labor which most gnomes resent. However, if there is a practitioner of arcane magic within a halfling community, odds are excellent that it is a gnome.

Gnomes and halflings may interbreed, but their children are always the same race as their same-gender parent.

* Usually. That said, when a gnome goes psychopathic, it's a terrible sight to behold. Think "magic-wielding serial killer" or "deathtrap-builder in the Saw vein."