I usually envision gnomes as sneaky bastards who prefer to confound you with illusions, shoot you from a distance well outside your Reach, or perhaps sick a shitload of badgers on you (again, staying out of melee range). Hell, if you manage to close with one and slap it upside the head, they can turn invisible for a turn and leg it.
Which makes this Winning Races article very strange and refreshing since its all about pimping out gnome defenders while still maintaining that sneaky element.
There are two paragon paths, one for paladins and one for swordmages. The paladin one is called Gnome Nightcloak, and as a twist requires training in Stealth. Kind of odd for a heavily armored character to be skulking about, but since the level 11 feature lets you add your Cha mod to Stealth it helps alleviate the heavy armor penalty. The concept behind it evokes the image of a 3-foot tall Bat Man archetype...I cant decide if thats cool, creepy, or comical.
The other two class features let you grant resistances to allies when you use fade away, teleport when you spend an action point, and deal bonus damage against a marked target if they also grant you combat advantage (kind of like Sneak Attack by another name).
Shadowstrike rebuke is my favorite prayer in the mix, and as a plus its also your level 11 attack. It lets you teleport and smack a baddy that hits/misses an ally like a swordmage can, but also pushes and sanctions them.
Cover of night lets you render an ally invisible. Simple but useful, and its got good range to boot.
Tumultuous shadowflow deals a crapton of damage and imposes an attack penalty in addition to preventing a target from benefiting from concealment for the entire encounter.
I greatly prefer the Phantasm Guard, which lets you emphasize illusion magic with the standard swordmage fare to get the feel for a decidedly "gnome" approach: you gain concealment against marked targets, regain fade away with action points, and get a Will kicker when you hit things with the psychic keyword (meaning that I can see lots of multiclass swordmage/wizards with illusion attacks).
Wave of dread is a close blast attack that deals psychic damage and dazes, and only hits enemies.
Nightmare landscape makes an area difficult terrain for enemies and grants all allies concealment. It lasts for the whole encounter wihtout requiring a sustain, but you cant move it. Still pretty fucking cool.
Illusory host also creates a zone effect, causing enemies caught within to grant combat advantage and letting you hit enemies that shift or attack allies without hitting you as well. Kind of like a "mark zone".
There are four feats that let you ignore the Speed penalty from heavy armor (but not plate), cause targets affected by divine challenge damage to grant combat advantage, grant concealment to allies you use lay on hands or virtue's touch on, and the long overdue Gnome Weapon Training: you can use all light blades and picks, and get a damage bonus to top it off.