Unless the galactic Golbats are giving you trouble, don't use Gardevoir. It sucks balls in platinum and it doesn't help you in any noteworthy battle. Even Maylene would prove to be a problem for her because two of her mons are neutral to psychic. Gardevoir would just horribly lose especially in the elite four against Aaron. If you wanna use Gardevoir, use it for the fun of it and only when you have a really sound team. Gardevoir horribly sucks in platinum against most noteworthy battles
If you already have Infernape, then just dump Machoke. You don't need double fighting types especially since Infernape is so much more faster, that is unless you are able to evolve it somehow and in which case, I guess Machamp is good but I still wouldn't recommend it. There is a reason why Infernape is considered one of the best mons from gen four
Use something stronger like Garchomp or Gyarados. Those two are super strong in platinum, especially Gyarados where you just dragon dance three times and one shot nearly everything. Magikarp is everywhere and you can get Gible as soon as you have the Eterna badge
But bottomline, from my own experience, Gardevoir sucks balls
