Gnomes are related distantly to fey creatures, and as such they have an innate tie to nature. They also are known to have very quirky personalities, and so they are often one of the easier races to roleplay, as roleplayers generally use their characters to fill a character role that they wish they could fill themselves, and this lends to character personalities that are on the more extreme ends of eccentricity. Gnomes have some innate magical abilities that can be useful in some campaigns, but are completely useless in others, so choose your alternate race traits wisely!
Ability Scores: Gnomes are hearty and easy-going, gaining a +2 to Constitution and Charisma, but their small size hurts their Strength score. This lends them well to essentially any type of spellcaster, but especially those that use Charisma as their main stat such as Sorcerer and Oracle. The boost to Constitution will be a help to characters of any class.
Size: Gnomes are Small, which is definitely a double-edged sword. They get a +1 size bonus to AC, which can be a huge help, and a +1 size bonus on attack rolls, which is wonderful. They also get a huge +4 size bonus to Stealth checks. The -1 to CMB and CMD hurts, but it’s not a huge deal. The real issue with small size comes from the next racial trait...
Speed: Gnomes have a base speed of 20 feet, and unlike dwarves, they are still subject to encumbrance. With a -2 to Strength, you’d be surprised how easy it is to encumber your gnome character to the point of being unable to move, so inventory management is of HUGE importance to any gnome.
Defensive Training: Though situational, a +4 to AC that stacks with absolutely anything can be a serious life-saver. Giants, watch out for those little creatures around your feet, they’re hard to hit!