If the Advanced Race Guide is allowed, I'd recommend the Fell Magic and possibly Knack with Poison alternate racial traits to start.
You may want to consider the Undead sub-school from Advanced Player's Guide; possibly even an arcane bomber (Ultimate Combat) with a two-level dip into alchemist to pick up Poison Use and the Concentrate Poison discovery. Alternately, since gnomes get a Cha bonus instead of an Int bonus, you might want to be a sorcerer instead of a wizard; the Accursed, Shadow (Umbral), Starsoul (Void Touched), and of course Undead (Sanguine) bloodlines (possibly using the wild-blooded archetype in Ultimate Magic) may be thematically appropriate.
Consider the dirge singer bard. It uses your cha and you get a bonus necromancy spell known for each spell level.