Okay... I'll help you out a bit:
Imagine a rabbit warren. Large amounts of corridors shooting off of main rooms. Everything a tight squeeze for creatures larger than Small size other than receiving rooms. Doors lack normal locks; instead, counterbalances exist which take small stones or weights (less than a pound) to trigger their apparatus (think prestidigitation effects). Protected areas are trapped using simple weight-sensitive pitfalls or other traps which trigger when any above 100 lbs. passes over them (the average weight of any two gnomes crossing the space together). Plenty of alchemical effects (thunderstone, tanglefoot bags) are in presence as protection, alongside strong illusions, mirrored passageways, strange acoustic effects...
If there are talented spellcasters, tricky uses of mirage arcana occur, and every night the 'visitor's chamber' is sealed via transmute mud to rock and/or stone shape. Gnomes often will create artificial settlements above their own... these settlements are usually lived in by allies whose elders know of the gnome's existence and protect them in trade for their wares. False wells (serving as air-ducts) and abandoned homes that open to large escape tunnels can be found inside of these settlements; due to their natural affinities and mutual love of knowledge and mercurial natures you may find halflings, kender, any number of fey, or dwarves living near the Little Folk. Gnomes provide a service as a buffer zone and refuge during attack, and their 'upper neighbors' can rely on gnomes talents in alchemy, illusion, and similar gifts in trade.
Gnomes find rarer allies in sylvan creatures, who wish to protect their homes from blight and can trust gnomes to keep to themselves and maintain their underground existence while assisting in spreading their landscape. Many dryad or fossergrim (?) would find themselves lucky to have gnomes living beside them; their natural talents in the earth keep the land diverse, and their sneakiness and skills would allow them to serve well as keepers of a glade.
More urban gnomes usually hold to their cave-dwelling roots (ha) but maintain homes. Unlike goblins who may hide in sewers or slums, gnomes create intricate passageways and workarounds between their homes (both mundane and magical). With their knowledge of the underground and ability to speak with burrowing animals, they easily shape their worlds. Usually gnomes live in a communal sort of existence, though gnomes may have several communes within the city.
Each group sells different wares, but they are still connected by their tunnels or other means of transport. An urban gnome commune would have a dozen secret passages, tricky little spaces, dumbwaiters which descend stories below ground, and most likely a caster with dimension door (or the coat of the mountebank) which would allow them access to the 'deep tunnels', which are the connections between each house. 600+ feet below the surface, these tunnels are made for quick evacuation and transit between communes, and there are large storehouses or meeting rooms beneath each gnome house which has enough food, water, and beds for a normal retinue to eat for a few days.
These cellars are checked every three days or when a dispatch is missed, and anyone inside of the cellar is brought up into the main house. Some houses equip a special 'Deepkeeper', a gnome with fighting talent and access to some method of bringing others with him, with a permanent mental alarm for the area. When the alarm is triggered, the Deepkeeper calls his allies and makes his way into the cellars to take care of business or shepherd those who have come to the main house.
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There are a few ideas; take them as you will.
Slainte,
-Loonook.
