Can you use Gentle Repose on an Undead?
disclaimer - my explanation deals more with "right, but how?" part of this, and can(should) be ignored if for someone "Magic!"+page number is enough ;-)It does need corpse, but I guess it depends on how you see the walking dead. IMO a lich body is by all means dead even if it's still animated by his arcane powers, and thus could be targeted by gentle repose spell, especially that apparently it just fends off germs shields from detrimental influence of oxygen. Now, trying to appear forever young by casting it on live body still wouldn't work, seeing how we need oxygen and the germs (yay ingestion!). On the other hand, since people raised after being under this spell do not immediately drop dead from suffocation, we could assume that any body-influencing spell cast after GR cancel it's preserving effects.
Even if we focus on the "a walking cadaver is not a cadaver, he's just cadaver-ish" line of thinking, you could have the cleric that is transforming you cast a permanent GR on your body before turning you to a lich.
However. The process of lich creation might be detrimental to the body by itself, as it's a case-by-case thing. Casting this spell on your body could very well prevent successful transformation, leaving you in an awkward spot.
All in all one can rule-lawyer it either way. Ultimately it's DM's call, but it's not without a merit, and could be fun.
ncG1vNJzZmivp6x7prrWqKmlnF6kv6h706GpnpmUqHykrc1msKitXarApnnGnqWtpJViv6a8zqycZqeeYq6vedSnm56ZlGN%2FeYSSbm9o