It’s also mentioned on one of the pages within the thread. 
But the Proboards search is pretty bad tbh, so it’s not too surprising.
I made that list in June 2019, so it might contain some errors, and it’s also fairly outdated. Lmk if you want me to update it (and I assume you’re only interested in English/Japanese Gengar cards, or all languages? And including other cards featuring Gengar in the artwork or only actual Gengar cards?)
Greetz,
Quuador