(Note: Gallade does exist, obviously, but you can't get a Dawn Stone until you defeat Wally at Victory Road.)
Gardevoir Pros:
-Solid Special Attack
-Good Special Defense
-Wide movepool
-Can Mega Evolve
Cons:
-Not the fastest
-Physically frail
-Psychic/Fairy makes it weak to some common types in the E4, Rivals and Champion
Gardevoir
Ability: Synchronize or Trace, either is good
-Calm Mind (learned at lv. 26)
-Psychic (Learned at lv. 31)
-Thunderbolt (TM given to you by Wattson)
-Moonblast/Dazzling Gleam (Learned at lv. 62/TM found on Route 123)
With these moves, Gardevoir can have supereffective damage against:
-Brawly's entire team
-Winona's team except for Tropius
-Wallace's entire team except for Wiscash
-Brendan/May's Wailmer, Wailord and Swellow (You chose Mudkip, correct?)
-Wally's Altaria and Gallade
-Maxie's Mightyena
-Archie's Mightyena and Sharpedo
-Tons of Aqua/Magma grunts
-Sidney's entire team
-Drake's entire team
-Steven's Skarmory
It also provides strong neutral damage, and coverage for tons of random trainers.
So yes, I do believe Gardevoir is worth your time.