Well, I've Got A Few Options.
1: As Gardevoir is A Special Tank, You Might Want A Physical Tank To Counteract with it. I'd Suggest Swampert Or Torterra.
#2: To Cover Gardevoirs Weaknesses, And Mix It With A Physical Pokemon For Some Mix, STaraptor Might Be Nice Covering All But Ghost(Brave Bird, Double Edge, Close Combat, Roost)
Or For A WAll That Mixes Gardevoir Gardevori with Bulk, Swampert or Umbreon.