As mentioned before, Gardevoir is in RU currently. This should be a pretty big indicator that Gardevoir probably isn't viable in OU.
This is Gardevoir's recommended set from Smogon.
Gardevoir @ Choice Specs
EVs: 252 SpA / 4 SpD / 252 Spe
Using this set (generally accepted to be Gardevoir's best set) Gardevoir is destroyed by some very common attackers. (For all of these calcs, I'm using the most common sets.)
252+ Atk Excadrill Iron Head vs. 0 HP / 0 Def Gardevoir: 420-494 (151.6 - 178.3%) -- guaranteed OHKO. (This Excadrill has Leftovers)
252 SpA Choice Specs Dragapult Shadow Ball vs. 0 HP / 4 SpD Gardevoir: 288-342 (103.9 - 123.4%) -- guaranteed OHKO.
However, Gardevoir does have some wallbreaking potential.
252 SpA Choice Specs Gardevoir Moonblast vs. 252 HP / 4 SpD Clefable: 246-291 (62.4 - 73.8%) -- guaranteed 2HKO after Leftovers recovery.
252+ SpA Choice Specs Gardevoir Psyshock vs. 252 HP / 252+ Def Toxapex: 224-266 (73.6 - 87.5%) -- guaranteed 2HKO after Black Sludge recovery.
Even if these seem underwhelming, a lot of special attackers can't boast the same.
Gardevoir should be used as a wallbreaker and only be switched into a resisted attack or a Pokemon that is Choice locked. It best fits on offensive builds that benefit from Gardevoir's ability to break past bulky offense teams.
Basically, Gardevoir can be good and function well, but it's niche and outclassed.
Smogon article on Gardevoir
Pikalytics Gen 8 OU
Pikalytics Damage Calculator