I would keep everything exactly the same, except:

The gnome becomes a Lich when it creates a phylactery. The phylactery is a magic item that grants resistance to necrotic damage, vulnerability to radiant damage, and gives the character an additional descriptor of 'undead'. After the PC dies while the phylactery exists, the player will resurrect within 1d4 days (or whatever you would like) wherever the phylactery is. The phylactery is noticeably magic. An evil creature can attempt to control the PC if it has the phylactery with a DC25 Arcana check.