Does your DM allow you to take LA on one side of the gestalt progression and class levels on the other or would you have to give up 8 full gestalt levels?
Since your DM was accommodating on the sunlight issue, it may be worthwhile to ask if you can use at least part of the Pathfinder version of the undead type. In Pathfinder, undead templates don't change your class HD to d12s and undead racial HD are d8s, but all undead get to use charisma in place of constitution for all purposes, including calculating HP.
If you just want to play an undead character, necropolitan (Libris Mortis) is a 0 LA template that makes you undead and doesn't really change anything else. Dread necromancer gives you the lich template for free at 20th level in addition to full spontaneous casting, but with a limited spell list. Walker in the waste (Sandstorm) gives you the dry lich template at 10th level and advances your spellcasting 8 out of 10 levels.