You select the monsters when you activate “Different Dimension Gate”.
If one player has no monsters on the field, you cannot activate “Different Dimension Gate”.
The controller of “Different Dimension Gate” selects both monsters.
A monster removed with “Different Dimension Gate” doesn’t take up a Monster Zone, but if there are no vacant Monster Zones when it returns to play, it is sent to the Graveyard.
When a monster removed with “Different Dimension Gate” is sent to the Graveyard with “Miracle Dig”, the monster won’t return to the field when “Different Dimension Gate” is destroyed, but the other monster removed with “Different Dimension Gate” will still return to the field.
When a Spirit Monster is removed with “Different Dimension Gate”, if it returns to play after the turn it was Summoned or Flip Summoned, it doesn’t go back to the owner’s hand.
When “Different Dimension Gate” is removed from the field, but not destroyed, because of “Giant Trunade” or “Emergency Provisions”, the removed monsters cannot be returned to play.
When your opponent’s monster on your side of the field that you control with “Change of Heart” or “Snatch Steal” is removed with “Different Dimension Gate”, it returns to your side of the field when “Different Dimension Gate” is destroyed, and then immediately returns to your opponent’s side of the field.
You can select a Monster Token when you activate “Different Dimension Gate”, but the token is taken off the field.
If your opponent chains “Ring of Destruction” to your “Different Dimension Gate”, only the surviving monster is removed from play and returns when “Different Dimension Gate” is destroyed.
When a Fusion Monster that was Special Summoned with “Summoner of Illusions” or “Magical Scientist” is removed from play with “Different Dimension Gate”, it is not destroyed or returned to the Fusion Deck when it returns to the field, and it no longer has “Magical Scientist”’s restriction that prevents it from attacking your opponent’s Life Points directly.
When you select “Guardian Kay’est” or “The Legendary Fisherman” as a target of “Different Dimension” gate, only the other monster is removed and returns to the field when “Different Dimension Gate” is destroyed.
You can only use “Different Dimension Gate”’s effect when it is activated, even though it is a Continuous Spell Card.
Netrep Rulings:
“Different Dimension Gate” may target a monster that is not affected by Spell Cards. That monster would not be removed from play by the effect of “Different Dimension Gate.”
Tokens may be targeted by “Different Dimension Gate”, but will not be returned to play if “Different Dimension Gate” is destroyed.
Monsters removed from play by the effect of “Different Dimension Gate” may be brought back into play by other means than the destruction of “Different Dimension Gate.”
If one monster affected by “Different Dimension Gate” has been returned to play, the other monster will still be returned to play when “Different Dimension Gate” is destroyed.