Gengar's weakness is partially due to its lack of speed, it is a stage 2 deck that usually can't use rare candy because it runs spiritomb as a starter. Because gengar prime barely does any actual damage, unless the opponent definitely has a pokemon in their hand to hurl into darkness gengar doesn't do much. Players have already learned that they need to keep their pokemon out of their hand, and if you can do that successfully then gengar isn't very threatening. However, the answer to that problem is hand disruption; spiritomb from triumphant is a really good option for gengar, because it forces the opponent to shuffle their hand and draw six new cards, and of course they can't control what pokemon appear in their hand that way. Mr. Mime from call of legends lets you know if your opponent has a pokemon to hurl, and if they don't then you can either spiritomb, giratina (let loose), looker's investigation, judge, or the worst: seeker. Seeker is a sure-fire way to get a pokemon into their hand to hurl.
This being said, gengar still needs the stadium in order to win the game. Dialga can deafen and buy some time by preventing them from playing stadiums. Overall though, I think luxchomp can still beat this deck down. It's fast enough that you can take out those gastlys and haunters before they become gengars, and it can take cheap prizes quickly from benched techs. It's easy to not have pokemon in your hand if you keep collectors and sp radars instead, only using them when you plan to play them down right away. Plus, gengar is weak to dark and with an absol G tech luxchomp can one-shot it with the help of either expert belt or crobat + poketurn.
tl;dr- Luxchomp is still better than gengar. However, I think if lostgar ran curse gengar from arceus it would have a better match-up. It gives you the ability to do damage as well as hurl, and if you shadow skip into gengar prime then its pokebody is activated and the knocked out pokemon goes to the lost zone.