Attacks » Dark Call GX
Does Umbreon GX's "Dark Call GX" attack have to discard 2 energy from a single Pokemon, or can I discard 1 energy from two separate Pokemon? Or does it mean to discard 2 energy from each of my opponent's Pokemon??
The term "Pokemon" in the attack is plural, but it doesn't mean all of your opponent's Pokemon. You may discard 2 energy from one Pokemon, or you may discard 1 energy each from two Pokemon.
Source: TPCi Rules Team (2017-01-26), TPCi Rules Team (2017-06-27)
Attacks » Dark Call GX | Attacks » Hyperspace Fury | Gameplay » Retreating
Can I use Umbreon-GX's "Dark Call GX" attack to discard 2 Double Colorless Energy?
Yes, you can select 1 Energy of each Double Colorless Energy to discard both of them. This applies for discarding your own Energy for an attack and for retreating as well. So if you used Hoopa-EX's "Hyperspace Fury" attack or paid its Retreat Cost of 2, discarding 2 Double Colorless Energy would be legal options.
Source: TPCi Rules Team (2017-01-26), TPCi Rules Team (2017-06-27)