Here is a list on the Berry's effect on Natural Gift.
Arceus will NOT get STAB. If you look at the link, Natural Gift with Sitrus Berry is Psychic Types. So it's Psychic type. Arceus must hold a Chilan Berry to have any STAB on Natural Gift at all, as Chilan Berries make Natural Gift Normal Type.
The move is almost like Hidden Power, just that Berries, not IVs, determine type and power.