Although I don't understand why it has Psychic weakness (in the video games, it's neutral to Ghost, double resistant to Psychic and immune to poison), I think it's not that kind of problem.
Gardevoir isn't able to OHKO unless it plays PlusPower and doesn't use DRE. Bronzong does 100 to Gardevoir (including Body) and OHKOs Gardevoir Lv.X. You may need DRE by yourself for Bronzong, but in case you play a deck focused around Bronzong and Aerodactyl, you can expect any Gardevoir to be already damaged so Bronzong OHKOs it no matter what.
Unfortunally, Plox maybe could be the only deck beaten by Bronzong/Aero/Omastar. Most of the other meta decks don't have that many PokéPowers in play and aren't damaged very much by Bronzong while almost anything can 2HKO it.
GG/Plox can stop Bronzong as well as Aerodactyl using LatiLock, but this would mean there is absolutely no space on the bench for any Dusknoir or Omastar techs.