I think you can nearly use the Halfling Druid substitution level as is.
Notably, the spontaneous casting of jump, spider climb, protection from energy, freedom of movement, tree stride, SNA6, SNA7, SNA8, and SNA9 works fine, the first five spells affecting the animal companion as well as the gnome druid.
The other first-level ability I'd give to gnome druids is the ability to prepare and cast any sorcerer/wizard illusion spell, as divine spells. However, he loses access to a number of standard druid spells:
[1] entangle, longstrider.
[2] barkskin, flaming sphere, fire trap.
[3] contagion, diminish plants, spike growth, and wind wall.
[4] antiplant shell, blight, dispel magic, flame strike, giant vermin, ice storm, and reincarnate.
[5] baleful polymorph, death ward, hallow, insect plague, unhallow, wall of fire, and wall of thorns.
[6] antilife shell, fire seeds, greater dispel magic, and repel wood.
[7] animate plant, changestaff, creeping doom, firestorm, and sunbeam.
[8] finger of death, sunburst, and whirlwind.
[9] regenerate and storm of vengeance.
Finally, the gnome gets Burrowing Animal Companion: a +3 bonus on his druid level for purpose of havign animal companions with a burrowing speed.
That's it for level 1.
Next sub level is level 5: the halfling druid's Undersized Wildshape is as valid for a gnome as it is for a halfling.