Hi! And welcome to the game
If you're looking for a way to start the game but not spend too much on them try playing with Night March or Medicham PRC Ancient Trait (what I'm currently playing). Theyre relatively cheap and pretty good (Night March being the better one). But since your question was about a Gengar EX deck, please read on.
I'd like to give you an actual list but that would take too much time because in my view there are a few ways you can run a Gengar EX Deck. Instead I'll just give you a bunch of possible partners and cards that you can use; just to give you ideas on how you want to build your deck. I will also limit this to mostly XY-on cards; a highly likely XY-on rotation might set you back a little especially if you have cards from BW sets (just a note on the rotation, I heard from a friend that its XY-on in Japan)
But firstly, I believe that 3 Gengar EX is enough. it's the core of the deck after all. You would want to focus on its second attack which basically does what Donphan currently does, and that is hit and run (with the addition of being able to poison).
Second, supporter cards are your core. You can check out other lists on this forum and they'll tell you run 4 Sycamore, 4 N, 2 Lysandre, etc etc. Just take a look around and you'll get the idea.
Okay on to the list of cards
M Gengar EX.
- In my opinion I think you should only run one of this. It's not the main attacker of the deck, but it can copy the attacks of others and has higher HP. That's something to keep in mind. The only problem is that it's a mega; it does have a spirit link, but I wouldn't devote that much deck space for it.
Dragalge FLF
- Its ability prevents poisoned pokemon from retreating. Since you're already poisoning your opponent's pokemon, why not let them stay there. This might seem a little bit weaker now though because Virbank City Gym is most likely going to leave the format.
Trevenant XY
- This card is seeing much more play now that Roaring Skies is legal. Basically if its active your opponent can't play Item Cards. That is a powerful ability, combined with Dragalge, they can't switch out their active pokemon.
Robo Substitute
- This card gives your Gengar EX a shield. Basically its a wall and a very good wall. It is an item card but played like a 30HP pokemon and if its knocked out your opponent doesn't take a prize.
Dimension Valley
- This gives your Gengar EX a free first attack or 1 [C] energy less for its second. A must have in any (or most) Psychic Deck.
Mystery Energy
- This gives your Psychich Pokemon 2 less retreat cost. Trevenant has a horrible retreat cost of 3. If you attach another one you're basically retreating for free to attack again with Gengar. If not, you can always attach a regular Psychic Energy and attack with Trevenant Instead.
Muscle Band
- Pretty standard. It boosts your attacks and stops Head Ringers and such being placed onto your Gengars.
That's pretty much what I have right now. I hope this helps