I thought I already did a Gengar list in the past, but apparently not…
Anyway, I currently don’t have much time to do a full list with all languages, so instead, here are the English ones missing in your list as requested:
- Fossil non-Holo unlimited with ©1999-2000 copyright instead of ©1999 from the Australian Red Logo pack (more info here) (PS: the Holo only exists as ©1999)
- Expedition Base Set MATCHPRINT (they are Color sample prints, and only 2 copies exist - more info here)
- The 34/119 Phantom Forces Gengar EX has a Jumbo print
And here are all the cards featuring Gengar that I know of:
- Art Academy Gourgeist (I just noticed there is also a Drifloon at the top of the artwork; do you also collect cards featuring Drifloon @queendrifloon ?)
- Blaine’s Quiz #1 from Gym Heroes
- Rocket’s Hideout from Neo Revelation
- Imposter Oak’s Reverse from Team Rocket
- Gengar Spirit Link from Phantom Forces
First one isn’t English, but figured I’d just include all of the ones I could find regardless of language.
Hope that helps. Good luck with your collection goals!
Thanks for the tag @mjisaacs and rofl at Quuadorized @melchior . 
EDIT: Ah, forgot about the recently discovered Fossil alternative holo pattern @aquaz mentioned. Need to remember that… (more info here, and the Gengar is shown in this video mark 32:10)
Greetz,
Quuador