Like most Pokmon cards, the value of Gardevoir-GX depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Burning Shadows)
Then as the set is opened more and metagames evolve, most new card prices decrease steadily until they hit a stable value
(If an opponent was dealt damage this turn, that creature enters with two additional +1/+1 counters on it.) Each other colorless creature you control has bloodthirst 2
Of course, Snapcaster Mage doesn't need help to be good, which might mean that playing it with Naban, Dean of Iteration is win-more, but this is where the power of being a 2/1 rather than a do-nothing four-mana artifact like Panharmonicon becomes clear
As the number of VMAX cards has increased, more powerful pocket monsters have been granted one of these game-changing forms
Its a Cheerios-style commander, making ample use of 0-mana artifacts like Mishra's Bauble and Spellbook to draw a bunch of cards before bouncing them back to your hand with Retract or Paradoxical Outcome to do it all over again