Gardevoir was initially NU and then it was placed in OU due to it's typing giving it innumerable advantages.
Gardevoir has gained a second chance in the OU community, as it has been retyped to Psychic / Fairy, giving it a fantastic x4 resistance to Fighting, an immunity to Dragon, and a neutrality to Dark, most notably Pursuit.
And gardevoir also has access to a good move pool. And trace is a good ability.
Now let's see why Mega Gardevoir is BL.
However, realistically it will see little use, as other Mega Pokémon like Alakazam, Charizard, and Lucario will find themselves on more teams. That doesn't mean that Gardevoir won't be viable, as it may still have the tools it needs to succeed in the OU tier.
Hope I helped:)