I think LoG will have all nighthaunt units plus some from LoN but keyword depending
I think they get
Deathrattle
Deadwalker
Deathmages
Maybe Malignant
Maybe deathlords
Lady O get deathlords keyword
LoG wizards with approriat Keywords get access to LoN spell lores maybe with Locus of shyish rule for all three spell lores
Different artifacts and general traits
Access to gravesite mechanic including endless legion command ability
Only thing i'm not confident about are Battle traits