So my count of 33 was incorrect it seems, and it’s 35 instead. 
Apparently I missed the Doublade Golurk is holding and the Honedge.
Up to and including gen 6 even. Gourgeist itself is gen 6, as well as Pumpkaboo; Phantump; Trevenant; Honedge; Doublade; and Aegislash. The only gen 6 Ghost-Type Pokémon that seems to be missing is Hoopa, but since it’s a legendary it was likely revealed a bit later on when rina already created this artwork.
Every other Ghost-Type Pokémon of gen 1-6 is accounted for.
Greetz,
Quuador