Imprison wins, at least for the particular Pokemon it's placed on.
Once the Imprison marker is placed, it doesn't depend on the original Power to maintain its effect. So anything with an Imprison marker on it still can't use Poke-Powers or Poke-Bodies, even after Glaceon LvX comes into play. (An analogous ruling exists in the Compendium for Lucario LvX's Stance vs. Cessation Crystal.)
But Gardevoir does need to use its Poke-Power to place Imprison markers. So no more Pokemon can get Imprison markers as long as Glaceon LvX is active.